High resolution, high frame rate, low power image sensor

ABSTRACT

An apparatus and method for determining a distance to an object using a binary, event-based image sensor. In one aspect, the image sensor includes memory circuitry and address decode circuitry. In one aspect, activation of a photodiode of the image sensor array by receipt of one or more photons is able to be used directly as an input to logic circuitry. In one embodiment, the image sensor includes photodiodes operating in an avalanche photodiode mode or Geiger mode. In one embodiment, the imaging sensor includes photodiodes operating as thresholded integrating pixels. The imaging sensor can be fabricated from one or more substrates having at least a first and a second voltage portion.

REFERENCE DATA

This application is a continuation of U.S. patent application Ser. No.14/680,906, Attorney Docket SSIC-0030, filed Apr. 7, 2015, and claimsthe benefit of U.S. Provisional Application No. 61/976,435, filed Apr.7, 2014, U.S. Provisional Application No. 62/072,320, filed Oct. 29,2014, and U.S. Provisional Application No. 62/072,327 filed Oct. 29,2014, the benefit of the earlier filing dates of which is hereby claimedunder 35 U.S.C. § 119(e) and the contents of which are furtherincorporated by reference in their entirety.

TECHNICAL FIELD

The present invention relates to the field of integrated circuits. Inparticular, it concerns an image sensor based on avalanche photodiodesthat measures distance of objects.

BACKGROUND

Ranging is a process whereby a distance from a detector to an object ofinterest may be determined. Conventional ranging systems may serve oneor more functions, including proximity sensing, remote sensing and/orthree-dimensional (3D) imaging. These systems typically include aninterrogation source (e.g., a light source) for illuminating the objectof interest, and an image sensor array (e.g., a complementary metaloxide semiconductor (CMOS) image sensor) for registering a return signalfrom the interaction of the interrogation source and the object.Conventional light interrogation systems may capture images using asequential frame-based image sensor and apply image processing andpattern recognition algorithms to the captured images. The algorithmsare typically based on knowledge of the projected broadcast patterns(e.g. structured light) or phase correlated beam sweeps of theinterrogation source. Typical image sensors require integration of manyphotons (e.g. 1000 or more photons) on each pixel in order to establishan optical signal of adequate signal-to-noise ratio (SNR), due to thelimited sensitivity of each respective pixel. The SNR requirements ofthe conventional image sensor in turn create demands on the power of thelight source (e.g., the interrogation beam which may be, for example, inthe visible wavelength range and/or infrared (IR) wavelength range).Stated differently, the interrogation beam must be able to provide anadequate number of photons for detection, in order to extract meaningfulinformation regarding the scanned environment (or object).

An image sensor having increased sensitivity (e.g., one includingavalanche photodiodes) enables signal detection with fewer incidentphotons from the interrogation source, but also becomes more sensitiveto background noise. When the range to an object of interest is large,the photons from the interrogation source have a large time-of-flight(TOF). For a system based on reflected (or backscattered) illuminationfrom the object of interest, radiometric dissipation of theinterrogation source (by 1/R⁴, where R=the range, due to theout-and-back path) requires that the interrogation source power be largein order to provide an adequate SNR over the large distances possiblefor detection. In such scenarios the signal from the “scene” may beoverwhelmed by the background, (e.g., large radiation of a bright sunnyday) which causes activation of the photodiodes in the image sensorprior to receipt of the interrogation signal returning from the objectof interest. Traditionally, TOF ranging issues, especially powerconsumed (which typically scales with the distance to the interrogatedobject(s)), along with the size of other system components, limits theuse of small form factor devices, e.g., handheld devices and/orwearables, for applications such as virtual and/or augmented reality.Conventional TOF-based systems are too power hungry and slow (regardinglatency, and frame rate) to be practical for use in mobile phones,wearables, and other mobile applications.

In conventional systems, the whole image sensor array is sampled (allpixels) in order to develop a grayscale image. Achieving a high framerate at low power with that scheme is not possible, when usinghigh-definition (many pixels, e.g., 1280×720) image sensor—the datastream is too large. Further, image sensors having high resolutiontypically use a memory-inspired array address and readout architecture,which severely limits the temporal information available, this temporalinformation being related to the arrival time of photons to the sensor.Massive amounts of pixel data must be sorted to find the events ofinterest in the conventional image sensor, a process that can be verypower-inefficient. Further, considering the great speed of light(approximately 0.3 meters per nanosecond), even a system capable ofachieving fine timing resolution, e.g., 0.5 nanoseconds, would belimited in spatial resolution to 0.15 meters, which may be toounresolved for many applications.

SUMMARY

This Summary is provided to introduce a selection of concepts in asimplified form that is further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter.

In one aspect, embodiments according to the present disclosure providean apparatus and method for determining a distance (e.g., range) to anobject. The apparatus includes an interrogation source for illuminatingan object, and an image sensor including highly sensitive photodetectordevices to detect illumination reflected from the object and to developa signal from the detected illumination. An analysis circuit operatingwith the interrogation source and the image sensor is able to determinea distance to the object based on triangulation derived from the imagesensor signal, corresponding to the received illumination reflected fromthe object. In an embodiment, a subset of photodetector devices in theimage sensor is activated in concert with activation of theinterrogation source, this activation defining one distance measurementperiod of the apparatus, a distance being determined in this period by alocation of the particular photodetector device in the image sensor thatdevelops a signal in response to incident reflected illumination. In anembodiment, distance to an object is determined based on an amount ofcharge stored on photodetector devices during a measurement period inresponse to incident reflected illumination.

According to embodiments of the present invention, a system is providedfor measuring a distance to an object. The system includes a photonicinterrogation source that is configured to emit an interrogation beam,at a controlled angle. The interrogation beam is for illuminating theobject. The system includes an image sensor that is coupled to andoperates in conjunction with the interrogation source. The image sensoris configured to receive a reflected beam generated by illumination ofthe object (from the interrogation beam), and the image sensor developsa signal based on a characteristic of the reflected beam. The systemincludes an analysis circuit that is coupled to and operates inconjunction with the interrogation source and to the image sensor. Theanalysis circuit is configured to determine a distance to the objectbased upon the controlled angle of the photonic interrogation source andupon the signal generated by the image sensor.

An image sensor may be configured in several ways for function in aranging system, including image sensor architectures adapted fortriangulation to an object using the address of an activated pixel, aswell as those including time-of-flight information at the pixel level asdescribed herein. In one aspect the use of a highly sensitive photodiode(e.g., a SPAD) enables identification of a specific location in theimage sensor array where illumination is incident via reflection from anobject, with great spatial discrimination. The entire image sensor isable to be binary when using a latch triggered directly from a SPADfiring event. Alternatively, the highly sensitive photodiode can beadapted for use as a trigger in a pixel-level TOF 3D system, where acharge level of the pixel is proportional to the time of flight ofphotons from the interrogation source, to the imaged object, and back tothe image sensor.

An image sensor according to embodiments of the present disclosure canbe fabricated to have a stacked substrate orientation, where a topsubstrate (that is, the substrate upon which illumination is incident)includes photosensitive elements (e.g., avalanche photodiodes) and abottom substrate includes control logic and circuitry corresponding tothe functionality described herein.

Embodiments according to the present disclosure provide a 3D imagingsystem having a completely digital image sensor architecture, asdescribed herein.

In one aspect, a method of detecting an optical event includesinterrogating an object with a photonic interrogation beam, activatingan array of pixels substantially concurrent with activation of thephotonic interrogation beam, the pixels comprising avalanchephotodiodes, a voltage bias level of avalanche photodiodes correspondingto pixel operation in one of Geiger mode and avalanche mode; receiving areflected beam from the interrogating the object, at the array;generating a signal corresponding to a location of incidence of thereflected beam on the array, the signal corresponding to at least one ofa pixel array address and a time-of-flight (TOF) of the photonicinterrogation beam and the reflected beam; and determining a distance tothe object based upon the signal.

According to an embodiment, the interrogating is performed at acontrolled angle of the photonic interrogation beam, wherein aninterrogation source emitting the interrogation beam, and the array, aredisposed at a controlled distance from each other, and wherein thedetermining the distance is based on triangulating the controlled angle,the controlled distance, and an angle of incidence determined by thepixel array address of the reflected beam. In a further embodiment, theactivating the array includes shuttering, wherein shuttering includesactivating a plurality of subsets of the array in a time-varying manner,wherein a subset activation period for each subset of the plurality ofsubsets is based upon a value of the controlled angle and a rate atwhich the controlled angle is varied. In a further embodiment, thegenerating the signal is based on the receiving the reflected beam by apixel included in an active subset of the plurality of subsets, andfurther wherein any detection of the reflected beam by a remainder ofpixels comprised by the active subset is not included in the signal. Inan embodiment, the generating the signal includes generating a setsignal by a latch circuit of a pixel of the array in direct response tothe pixel receiving the reflected beam, wherein the set signal forwardsat least one of a row address and a column address of the pixel to atleast one of a column decoder circuit and a row decoder circuit. In afurther embodiment, the set signal is a positive voltage pulse developedby a ground-referenced photodiode current. In an embodiment, thegenerating the signal includes generating a column decode signal at apixel of the array in response to the pixel receiving the reflectedbeam, wherein the column decode signal forwards the column address ofthe pixel to a row latch circuit. In an embodiment, pixels of the arraycomprise at least one charge storage element, and further activating thephotonic interrogation beam and the array of avalanche photodiodesinitiates and terminates a changing charge level of the at least onecharge storage element during a time period between an emission from theinterrogation beam and the receiving the reflected beam from the object,and further wherein the charge level corresponds with the TOF.

According to an aspect of the present disclosure, an optical eventsensor for detecting an optical event includes an array of pixelsoperable to be activated substantially concurrent with activation of aphotonic interrogation source, the pixels including avalanchephotodiodes, a voltage bias level of avalanche photodiodes correspondingto pixel operation in one of Geiger mode and avalanche mode; wherein thearray of pixels is operable to receive a reflected beam from the object,at the array, and to generate a signal corresponding to a location ofincidence of the reflected beam on the array, wherein the signalcorresponds to at least one of a pixel array address and atime-of-flight (TOF) of the interrogation beam and the reflected beam,and wherein the array of pixels is operable to determine a distance tothe object based upon the signal.

In an embodiment, the object is interrogated at a controlled angle ofthe photonic interrogation source, and further the photonicinterrogation source and the array of pixels are disposed at acontrolled distance from each other, and the distance is determinedbased on triangulating the controlled angle, the controlled distance,and an angle of incidence determined by the pixel array address of thereflected beam. In a further embodiment, the array is operable toshutter, including activating a plurality of subsets of the array in atime-varying manner, wherein a subset activation period for each subsetof the plurality of subsets is based upon a value of the controlledangle and a rate at which the controlled angle is varied. In a furtherembodiment, the signal is generated based on detection of the reflectedbeam by a first pixel of an active subset of the plurality of subsets,and any detection of the reflected beam by a remainder of pixelscomprised by the active subset is not included in the signal. In anembodiment the signal includes a set signal by a latch circuit of apixel of the array, the set signal in direct response to the pixelreceiving the reflected beam, wherein the set signal forwards at leastone of a row address and a column address of the pixel to at least oneof a column decoder circuit and a row decoder circuit. In a furtherembodiment the set signal is a positive voltage pulse developed by aground-referenced photodiode current. In an embodiment, the signalincludes a column decode signal at a pixel of the array, the columndecode signal in response to the pixel receiving the reflected beam,wherein the column decode signal forwards the column address of thepixel to a row latch circuit. In a further embodiment, the array ofpixels includes a first substrate configured to operate at a firstvoltage and a second substrate configured to operate at a secondvoltage, wherein the first voltage is higher than the second voltage,and wherein the photodiode is included in the first substrate andwherein logic and control circuitry of the array of pixels is includedin the second substrate. In an embodiment, pixels of the array includedat least one charge storage element, and further wherein activation ofthe photonic interrogation beam and the array of avalanche photodiodesis operable to initiate and terminate a changing charge level of the atleast one charge storage element during a time period between anemission from the interrogation beam and the receiving the reflectedbeam from the object, and further wherein the charge level correspondswith the TOF.

According to an aspect of the present disclosure, a distance measuringmobile apparatus includes a processor; a memory operatively coupled tothe processor; a photonic interrogation source configured to emit aninterrogation beam for illuminating an object; and an image sensorincluding an array of pixels and corresponding charge storage elements,the image sensor operatively coupled to the interrogation source and tothe processor, the image sensor configured to receive a reflected beamgenerated by illumination of the object by the interrogation beam, andto develop a signal based on the reflected beam on the array, whereinactivation of the interrogation source and the image sensor initiatesand terminates a changing charge level of the charge storage elementsduring a time period between an emission from the interrogation beam anda reception of a reflected beam from the object, and further wherein thecharge level is comprised by the signal; wherein the processor isoperable to receive the signal and to output distance information of theobject based on the signal.

According to an embodiment, the array of pixels includes an array ofavalanche photodiodes operating in Geiger mode. In a further embodiment,the charge storage elements include a capacitor, the capacitorconfigured to terminate charging upon receiving a signal generated by aphotodiode of a corresponding pixel in response reception of thereflected beam.

While a highly sensitive, binary-like photodiode is enabled foroperation of image sensor architectures of the present disclosure,conventional (e.g., active) pixel architectures are also able to beutilized, for both image sensor modalities (e.g., for triangulation viapixel address, and for pixel-level TOF information). The active pixelrequires sufficient sensitivity to detect a meaningful number ofinterrogation source photons to develop a voltage signal. However, usinga voltage thresholding approach, digital image sensor architecturesaccording to the embodiments described herein may be realized withactive pixels—but when using avalanche photodiodes or SPADs, no senseamplifiers or analog-to-digital converters are required for the imagesensor.

Therefore, a range of image sensor architecture implementations iscontemplated—triangulation using a SPAD array; triangulation using anactive pixel array; TOF using a SPAD array; and, TOF using an activepixel array. Each of these architecture implementations provides manydegrees of freedom in controlling the operation of an imaging sensor.The ability to vary the timing between the activation of aninterrogation source and the activation of an image sensor providescontrol of a depth of field for an interrogated field of view. Controlof the output power of the interrogation source enables improvements insignal-to-noise over ambient light, as well as the ability to tune theinterrogation source power for the desired range ofinterrogation—objects interrogated closer to the image sensor requireless illumination than those located further away. The output framerateof the image sensor may be varied by controlling the number of pixelsthat are active for a given measurement. These features of architecturesaccording to the present disclosure provide optimization to be made forpower consumption, framerate, and/or the image resolution of a 3Dimaging system.

In the context of the present disclosure, the term “illuminating”generally refers to an electromagnetic wave in the infrared, visiblelight, and/or ultraviolet range.

In the context of the present disclosure, the term “multiplexer” refersto an electrical or opto-electrical switch with selection inputscontrollable by either electrical or photonic signals.

In the context of the present disclosure, the term “array” refers to anarrangement of elements in R rows and C columns, where R and C are eachgreater than or equal to unity. For example, an array can be a completeset of photodiodes in an image sensor, and/or pixels in an image sensor.Further, R and C may have equal or unequal value, and a spacing betweenrows and between columns may be equal or unequal. The terms “sub-array”and “subset of the array” refer to a group of array elements fewer innumber than the full array, which may be either contiguous ornon-contiguous.

The foregoing is a summary and thus contains, by necessity,simplifications, generalizations and omissions of detail; consequently,those skilled in the art will appreciate that the summary isillustrative only and is not intended to be in any way limiting. Otheraspects, inventive features, and advantages of the present invention, asdefined solely by the claims, will become apparent in the non-limitingdetailed description set forth below.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthis specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention:

FIG. 1 depicts an exemplary optical interrogation system, in accordancewith embodiments of the present invention.

FIG. 2 depicts an exemplary avalanche photodiode pixel, in accordancewith embodiments of the present invention.

FIG. 3 depicts an exemplary current-voltage relation of a single-photonavalanche photodiode, in accordance with embodiments of the presentinvention.

FIG. 4 depicts an exemplary optical interrogation system operating in anenvironment including a background radiation source, in accordance withembodiments of the present invention.

FIG. 5 depicts an exemplary schematic of an image sensor architecture,in accordance with embodiments of the present invention.

FIG. 6 is an illustration of an environment having multiple objects fordistance determination, in accordance with embodiments of the presentinvention.

FIG. 7 is an illustration of an environment having multiple objects fordistance determination, in accordance with embodiments of the presentinvention.

FIG. 8 is a flowchart illustrating a method of determining a distance toan object, in accordance with embodiments of the present invention.

FIG. 9 depicts an exemplary optical interrogation system integrated witha host device, in accordance with embodiments of the present invention.

FIG. 10 depicts a block diagram of exemplary pixel circuitry inaccordance with embodiments of the present invention.

FIG. 11 depicts an exemplary pixel circuit, in accordance withembodiments of the present invention.

FIG. 12 depicts an exemplary pixel circuit including buffer and decoderelements, in accordance with embodiments of the present invention.

FIG. 13 depicts exemplary circuit logic for image sensor columnself-timing and readout ripple-through functionality, in accordance withembodiments of the present invention.

FIG. 14 depicts a block diagram of connected pixel circuitry, inaccordance with embodiments of the present invention.

FIG. 15 depicts a schematic of an image sensor architecture, inaccordance with embodiments of the present invention.

FIG. 16 depicts a schematic of an image sensor architecture, inaccordance with embodiments of the present invention.

FIG. 17 depicts a schematic semiconductor device having at least twolayers with portions configured for operation at a first and a secondvoltage, in accordance with embodiments of the present invention.

FIG. 18 depicts an imaging sensor array having a stacked wafer design,in accordance with embodiments of the present invention.

FIG. 19 depicts processing steps for fabrication of an imaging sensorarray having a stacked wafer design, in accordance with embodiments ofthe present invention.

FIG. 20 depicts an imaging sensor array having a stacked wafer designand a reduced pixel pitch, in accordance with embodiments of the presentinvention.

FIG. 21 is a flowchart illustrating a method of fabricating asemiconductor device, in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION

In one aspect, embodiments of the present disclosure provide a systemhaving an interrogation source operatively coupled to an image sensor.The interrogation source (for example, a laser tuned to an infrared (IR)wavelength) can be configured to illuminate an object or objects withinthe field of view of the system, and the image sensor is configured todetect reflected illumination from the object of interest. The imagesensor includes an array of highly photo-sensitive detectors (e.g., anarray of avalanche photodiodes). In an embodiment, an avalanchephotodiode comprises a p-n junction device operating beyond itsbreakdown region, operating as a single photon avalanche diode (SPAD),such that an incident photon is able to cause a self-sustained avalanche(e.g., an avalanche photodiode operating in so-called Geiger mode). Inan embodiment, the image sensor responds to one (or a few) incidentphotons and develops a signal indicating photon detection at aparticular image sensor array address. Stated differently, reception ofa few photons by a photosensitive element of a pixel is adequate fordetection by the pixel, leading to a reduction in the number of photonsrequired to detect an optical event when using such a pixel (as comparedwith a pixel operating in a conventional manner, with photodiodesintegrating hundreds of photons over a time period in order to detectillumination and develop a signal). By accurate position informationderived by a pixel address, along with a known interrogation sourceangle and distance to an image sensor, a distance to an object can bedetermined at great accuracy with minimal circuit complexity, cost, andpower consumption. Further, the image sensor is able to be configured tooperate by enabling time-varying subsets of image array pixels (e.g., arolling pixel sub-array), in concert with activation of theinterrogation source, in order to reduce both background noise andsystem power requirements. Use of such allows, for example, much loweroptical output power to be used in the corresponding light source of anoptical interrogation system, one possible advantage of lower opticalpower requirements being human eye safety.

It is appreciated that single- or few-photon event detection pixels areapplicable to various optical interrogation-based sensing modalitiesincluding, but not limited to, ranging, TOF and 3D imaging applications.In an embodiment, the individual pixels of an image sensor array areable to determine TOF information of received illumination reflected (orbackscattered) from the object(s) of interest. An optical package may beincluded on the system, which is able to filter the wavelength ofincoming radiation to the image sensor and/or function to focus incidentradiation upon the light-sensitive portions of pixel (e.g., improve fillfactor of the pixel). In an embodiment, the image sensor array isenabled to receive illumination in a synchronous manner with theactivation of the interrogation source. The system of the presentdisclosure is able to discriminate, over moderate distances, a photonburst received from the interrogation source rather than from backgroundnoise, in bright ambient light, with a minimum of on-chip circuitry,latency, and power consumption. FIG. 1 provides an overview of a rangingdevice according to an embodiment of the present disclosure. It isappreciated that an optical interrogation system utilizing such asingle- or few-photon pixel is able to be integrated into a host devicesuch as a mobile device (e.g. smart phone, tablet, etc.) or other hostdevices.

Reference will now be made in detail to several embodiments. While thesubject matter will be described in conjunction with the alternativeembodiments, it will be understood that they are not intended to limitthe claimed subject matter to these embodiments. On the contrary, theclaimed subject matter is intended to cover alternative, modifications,and equivalents, which may be included within the spirit and scope ofthe claimed subject matter as defined by the appended claims.

Furthermore, in the following detailed description, numerous specificdetails are set forth in order to provide a thorough understanding ofthe claimed subject matter. However, it will be recognized by oneskilled in the art that embodiments may be practiced without thesespecific details or with equivalents thereof. In other instances,well-known methods, procedures, and components, have not been describedin detail as not to unnecessarily obscure aspects and features of thesubject matter.

Portions of the detailed description that follows are presented anddiscussed in terms of a method. Although steps and sequencing thereofare disclosed in figures herein (e.g., FIGS. 8 and 21) describing theoperations of these methods, such steps and sequencing are exemplary.Embodiments are well suited to performing various other steps orvariations of the steps recited in the flowcharts of the figures herein,and in a sequence other than that depicted and described herein.

Embodiments described herein may be discussed in the general context ofcomputer-executable instructions residing on some form ofcomputer-usable medium, such as program modules, executed by one or morecomputers or other computing devices. Generally, program modules includeroutines, programs, objects, components, data structures, etc., thatperform particular tasks or implement particular abstract data types.The functionality of the program modules may be combined or distributedas desired in various embodiments.

By way of example, and not limitation, computer-usable media maycomprise computer storage media and communication media. Computerstorage media includes volatile and nonvolatile, removable andnon-removable media implemented in any method or technology for storageof information such as computer-readable instructions, data structures,program modules or other data. Computer storage media includes, but isnot limited to, random access memory (RAM), read only memory (ROM),electrically erasable programmable ROM (EEPROM), flash memory or othermemory technology, compact disk ROM (CD-ROM), digital versatile disks(DVDs) or other optical storage, magnetic cassettes, magnetic tape,magnetic disk storage or other magnetic storage devices, or any othermedium that can be used to store the desired information.

Communication media can embody computer-readable instructions, datastructures, program modules or other data in a modulated data signalsuch as a carrier wave or other transport mechanism and includes anyinformation delivery media. The term “modulated data signal” means asignal that has one or more of its characteristics set or changed insuch a manner as to encode information in the signal. By way of example,and not limitation, communication media includes wired media such as awired network or direct-wired connection, and wireless media such asacoustic, radio frequency (RF), infrared and other wireless media.Combinations of any of the above should also be included within thescope of computer-readable media.

In the following embodiments, a technique to perform high resolution,high frame rate, low power consumption ranging is described. Embodimentsinclude a method of determining a distance to an object including stepsof illuminating the object with an interrogation beam, receivingreflected illumination from an object using a highly photosensitiveimage sensor, and determining a distance to the object based on anaddress of an activated pixel using trigonometry, and/or TOF informationdetermined at the pixel level.

SPAD Control

In an embodiment of the present invention, avalanche photodiodes of animage sensor (including, e.g. local and/or global variable reversebiasing) may be used to increase sensitivity and reduce exposure timerequirements in ranging and 3D imaging applications. An avalanchephotodiode in Geiger mode (e.g., a SPAD) operates in very extremereverse bias condition, beyond natural reverse bias breakdown, and maybe held at such a voltage only temporarily until an event triggers it to(very rapidly) develop current. There is therefore a time factor whenusing such a device as a detector, as the SPAD cannot be maintained inthe extreme reverse bias condition for very long. The large currentdeveloped by a SPAD following an event trigger can be destructive, sooften a resistor is added to the diode in order to limit the current,such that once the SPAD does “fire” (e.g., rapidly generate current) itmay output current to a safe level in a short amount of time. Oncefired, the SPAD avalanche current is quenched by the resistor or otherload mechanism. The voltage at the SPAD may be buffered (and likewise, achange in polarity may be induced) for pixel operation. An example of anavalanche photodiode according to the present disclosure is depicted inFIG. 2.

A SPAD responds very quickly to a photon event, on the order ofpicoseconds. Factors such as the wavelength of incident photons,environmental temperature, the cell size (area) of the SPAD, and theextent of voltage bias (e.g., overvoltage), quantum efficiency, and darkcount rate (DCR) affect functionality of the SPAD. Once discharged, aSPAD takes some time to recharge (so-called “deadtime”), typically inthe approximate range of 20-40 nanoseconds. Typically, only a few tensof photons (approximately 20 photons, dependent on photon wavelength anddetector cell size) are sufficient to trigger the rapid currentgeneration of the SPAD.

In the present embodiment, a pixel comprising a SPAD is able to beconfigured for variable pixel sensitivity on a local and/or globallevel, and can be tuned according to ambient light and/or depth of fieldrequirements (i.e. the reverse bias voltage control can be used toadjust photodiode sensitivity according to exposure constraints and/orrequirements). In a system of this operation, there is a range limit setby the photodiode sensitivity. When operating in Geiger mode, a SPADpossesses a dark count rate (DCR), a measure of SPAD noise whose valueis dependent on factors such as the SPAD cell size, the temperature ofthe SPAD, and the overvoltage (e.g., the voltage level beyond breakdownthreshold). The DCR of the SPAD, in the absence of any illumination,provides a statistical measure of the length of time a SPAD may remainin Geiger mode before experiencing an avalanche event. In combinationwith ambient light, DCR provides a background noise signal that must bedistinguished from that of the transmission of the interrogation beam inorder for a meaningful signal to be developed. Therefore the DCR, whencoupled with ambient photons accumulated during the TOF, provides anestimate of a maximum range at which objects are able to beinterrogated.

In an embodiment of the present disclosure, an optical notch filter isused (e.g., at optics 135 of FIG. 1) to prevent excessive backgroundnoise photons from affecting the pixels, outside of the interrogationwavelength of interest. The interrogation source is able to be selected,or tuned, to emit wavelengths near the center of the optical notchfilter. In a further embodiment, the center wavelength of the opticalnotch filter is selected so as to minimize background radiation from thesun—for example, to have a center wavelength corresponding to a strongatmospheric absorption line, such as 850 nanometers or 940 nanometers(e.g., atmospheric absorption lines due to water). Such animplementation of the present disclosure would allow, for example,higher photodiode gain. A tradeoff of decreasing the width of theoptical filter is that a decrease is also had in the field of view forthe system. A tunable interrogation source, while potentially addingcost, may serve to further improve SNR. The total optical power, as wellas the wavelength, should be considered when operation is anticipated inenvironments where eye harm may arise. Additionally, the imaging framerate may be determined based on pixel sensitivity (i.e. exposure timerequirements) and/or readout rate and/or pixel array size. High framerate refresh can be employed in order to achieve adequately high Nyquistsampling rates of a moving object.

Embodiments of the present disclosure provide an image sensor havingextremely sensitive photodiodes, whereby distance determination to anobject is able to be made by a novel addressing scheme reporting thearray address of a pixel activation resulting from incidentinterrogation illumination. It should be noted that the presentinvention is not limited to SPAD technology, and that any photondetector of sufficient sensitivity can be used.

Ranging Via Integrated Triangulation

In one aspect, embodiments of the present disclosure provide a photonicinterrogation system for determining a distance to an object. FIG. 1 isa schematic illustration of an exemplary photonic interrogation device105 to determine a distance to an object, in accordance with anembodiment of the present disclosure. Interrogation device 105 includesan interrogation source 110 and an image sensor 115. Interrogationsource 110 and image sensor 115 are separated by a defined distance 120(as a non-limiting example, distance 120 is 5 centimeters). In anembodiment, distance 120 can be modified in a controlled manner.Interrogation source 110 is configured to emit an interrogation beam125, and image sensor 115 is configured to detect returned illumination130 from an object in the field of view of the optical device 105.Imaging sensor 115 includes an array of photosensitive diodes 140, forexample, avalanche photodiodes, which form pixels of image sensor 115.

In an embodiment according to the present disclosure, array 140 includesa SPAD pixel 140 a formed by at least two stacked wafers (orsubstrates). The SPAD pixel 140 a can operate in a linear avalanchemode, or in Geiger mode, based on the bias voltage. SPAD pixel 140 aincludes a photosensitive area 145 a, and pixel circuits in region 150 a(e.g., high voltage line, pixel input/output lines). According to anembodiment, pixel circuit elements corresponding to pixel circuit logicare formed on a separate substrate from the SPAD cell (e.g., thephotosensitive region formed on a substrate corresponding to p/njunctions). That is, a substrate having a plurality of SPAD cells can bestacked on top of a wafer having a plurality of logic and controlcircuitry (e.g., transistors, enable lines, etc.), as is describedherein. In an embodiment according to the present disclosure, array 140includes a SPAD pixel 140 b formed on one wafer, where photosensitiveregion 145 b can be adjacent to pixel circuit logic 150 b (e.g., highvoltage line, pixel input/output, TOF information storage element(s)).Pixels 140 a and 140 b can include at least one time-of-flight (TOF)element, that is, an avalanche photodiode operating in either linearavalanche or Geiger mode and that includes, for example, a storagecapacitor configured to store charge corresponding to a TOF value. Anoptical system 135 may be included in device 105, whereby optical device135 focuses and/or filters incident reflection onto image sensor 115.The optical interrogation device 105 may include an ambient light sensor(not shown). As shown in FIG. 1, interrogation beam 125 is directedtoward object 150, and returned illumination 130 falls upon image sensor115.

In an embodiment, interrogation source 110 is a laser, for example alaser having a resonant MEMs scanning mirror (e.g., a vertical cavitysurface-emitting laser (VCSEL). The interrogation source 110 is able toemit radiation at an IR wavelength, or at other wavelengths. In anembodiment, the mirror is resonant in the x direction (e.g., along imagesensor array columns), and stepped in the y-direction (e.g., along imagesensor array rows). A resonant scanning laser is able to sweep aninterrogation beam 125 across a field of view for the interrogationdevice 105. In an embodiment, the interrogation source 110 generates araster scan of high intensity photons (e.g., interrogation beam 125)from the laser. In an embodiment, in order to account for the resonantmotion of interrogation source 110 across the field of view, activationof the interrogation beam 125 is controlled in order to providesubstantially equal irradiation across image sensor 115 (e.g.,sinusoidal linearization of a duty cycle of interrogation source 110,based on interrogation source angle 111).

Activation of the interrogation source 110 in a collimated “spot” mode,at particular angles (e.g., interrogation source angle 111) and with adefined pulse period, serves to “paint” a grid image, one pixel at atime at the image sensor 115. A scanning rate of the mirror (andtherefore the laser beam) is an important factor in system timing,particularly pixel activation timing, as is discussed herein. Theinterrogation source 110 and the image sensor 115 are coplanar,constraining an image sensor 115 field of view of the scene to a sameplane as the device 105's field of view. Therefore, any return photons(e.g., returned illumination 130) will be in the same plane as theinterrogation beam 125. So, with regard to the array of image sensor115, the row having an expected photodiode excitation will be known,based on the angle 111 of the interrogation source 110. However, theparticular column within the row is not known, because the distance tothe object 150 (and therefore the return angle of return illumination130) is unknown. Object 150 may be at multiple distances/locationswithin the device 105 field of view.

The angle that the returned light 130 comes into the optics (angle 116)determines the column of the image sensor array having photodiodeactivation (e.g., the position within the row), and therefore thedistance (via triangulation). Lensing and optics (e.g., optics 135) ofthe device 105 including image sensor 115, will see a spot (solid angleof light) at a distance, up to a range limit of viewing. Photons fromthat spot (the spot on the object 150), hitting an aperture of the imagesensor 115, will be focused onto one array element (e.g., one pixel). Inan alternative embodiment, the interrogation source 110 operates in a“fan” mode, wherein a stripe of illumination is emitted. In anembodiment the stripe is substantially perpendicular to the axis ofinterrogation source movement (e.g., the resonance axis). In such anembodiment, returned illumination 130 may be incident upon an entirecolumn (or row) of array elements, parallel to the orientation of theinterrogation beam 125. Due to the features of the interrogated object(e.g., object contours), the returned illumination 130 may also returnnon-planar and be incident over several columns, corresponding to thecontours of the object. In fan mode, all photodiodes rows are active allat once. The output power of the interrogation source can be increased,to compensate for distribution over a whole line (all rows). The imagesensor 115 architecture, for such a device, is able to report a photonburst from the interrogation source 110 in bright sunlight, with minimumlatency, minimum power consumption, and high refresh frequency—all withminimum of on-chip circuitry. The distance determination to an object isdetermined, in an embodiment, using accurate position trigonometry viathe position of a firing photodiode in an image sensor 115 array (e.g.,the array address). As is discussed herein, due to background noiseconsiderations, embodiments of the present disclosure includephotosensitive diodes of the image sensor 115 activated for sensing in asynchronized manner (e.g., “gated”) with activation of the interrogationbeam 125 for a particular measurement period. A system as describedherein is able to provide high throughput via the high sensitivity pixelevents, reported according to the pixel address. Further, whileembodiments according to the present disclosure provide a means ofcalculating a distance to an object via a TOF calculation at the pixellevel, such information is not necessary for embodiments whereindistance is calculated according to trigonometry information (e.g.,triangulation). A positional approach (e.g., pixel address) is able toreduce the cost of a system, by removing the need for expensivecircuitry in the pixel to generate TOF information. Further, computationrequirements are reduced, as the pixel address may be found via, forexample, a lookup table. Additionally, a minimization in circuitry atpixel levels increases the resolution in the system for an array of thesame size, because the photon-sensitive portion of each pixel becomeslarger as a share of the total pixel (e.g., fill factor increased).Factors such as cost and size are of significant importance forapplications such as handheld and/or wearable devices.

FIG. 2 depicts a SPAD pixel architecture 140, according to embodimentsof the present disclosure. In an embodiment, enabling the SPAD pixel isimplemented by a word line and a bit line signal. The SPAD pixelincludes a photodiode 145. The SPAD pixel may include active and/orpassive quenching, via active- or resistive-load 220, respectively. TheSPAD pixel can include a buffer 225. In an embodiment, active quenchingis provided by an active load, the activation of which can be based onfeedback (e.g., rising edge trigger of SPAD avalanche current) from anoptional feedback quenching circuit 230. Quench time is able to be builtinto a multiplexer decode switch overhead time. In an embodiment, pixelrow and column driven reset is built in, enabling a reset of statisticalnoise (e.g., DCR, ambient light). In an embodiment, the high voltagesupply line is static (e.g., not switched), and the ground-referencedword line enable allows a high voltage current path.

FIG. 3 depicts a SPAD current-voltage characteristic 300, according toan embodiment of the present disclosure. The current-voltagecharacteristic 300 includes arrows indicating phases of SPAD operation.A reverse bias level Vgeiger is depicted, where reverse bias levelVgeiger is a reverse bias voltage beyond the breakdown voltageVbreakdown of a semiconductor p-n junction, and indicates so-calledGeiger mode of an avalanche photodiode (e.g., a SPAD). At voltage levelVgeiger, a SPAD will develop a current (avalanche of electrons) fromincident photon radiation, as well as thermal noise. SPAD gain (e.g.,electron multiplier factor) is substantially linearly dependent onovervoltage and area of the SPAD cell size. Likewise, the likelihood ofphoton and/or thermal activation of the SPAD (e.g., DCR) increases withincreasing overvoltage. Once avalanche current is initiated, the SPAD isquenched in order to prevent excessive current flow. Once the SPAD isquenched and has returned to a lower voltage bias, in order to preparethe SPAD for further activation, the SPAD voltage bias is returned toVgeiger. Typical timing values for a SPAD to breakdown (e.g., “fire”)are less than 1 nanosecond, while quench and reset are approximately20-40 nanoseconds. Quenching may be either passive or active, and in apreferred embodiment, active quenching is used to minimize quenchingtime and thereby increase the rate with which a SPAD generates multiplesignals.

FIG. 4 depicts a scene 400 including bright illumination, and includes aschematic illustration of an exemplary photonic interrogation device 105operating to determine a distance to an object 440, with a backgroundradiation source 401 (e.g., the sun) illuminating object 440.Interrogation source 110 emits interrogation beam 425 at angle 411,which is reflected (or backscattered) from object 440 to form returnedillumination 430 at angle 416. Returned illumination 430 is received byimage sensor 115 (located at distance 120 from image sensor 115), andforms the basis of the distance determination made by photonicinterrogation device 105. Concurrent with the illumination beam 425,radiation source 401 is constantly illuminating object 440, with a fluxof photons dependent on the output power of the radiation source 401—inthe case of the sun, radiation is broad spectrum. Photons from radiationsource 401 illuminate object 440, and form reflected illumination 435which is incident upon the image sensor 115.

The image sensor 115, which comprises highly sensitive photodiodes, doesnot discriminate photons from the interrogation source 110 from those ofthe radiation source 401. For an image sensor including avalanchephotodiodes operating in Geiger mode (e.g., including SPADs), anindividual diode responds to approximately 20 photons. The dark countrate of the SPAD, along with a large photon flux from radiation source401, will lead to quick firing of a SPAD within image sensor 215, whichmay be prior to receipt of returned illumination 430 for an object 440at moderate distance from device 105. Illumination of interrogationsource 110 on the object 440 (e.g., the transmit signal), along with theillumination of object 440 by the radiation source 401, e.g., the sun(with optical filtering, the sun's illumination at the wavelength beingsensed—e.g., the laser wavelength of interrogation source 110)—give thesignal-to-background noise ratio (SNR). It is therefore beneficial toactivate photodiodes of image sensor 115 with a timing based onactivation of interrogation source 110, in order to improve SNR byminimizing the number of ambient photons incident upon image sensor 115during measurement, as well as reducing overall DCR contribution tonoise at the image sensor 115 by reduced pixel activation time.

Concerning background noise, longer measurement lengths, withcorresponding larger TOF, lead to an increase in both ambient photonsreceived and in dark counts. Photons arrive at the image sensor 115 overthe entire TOF of the interrogation beam 425. In order to generate ameaningful signal for determining the range to object 440, the returnedillumination 430 must impart the requisite number of photons to activatea first enabled photodiode prior to activation by the ambient light andthe DCR. Since the interrogation source 110, which is typically a laser,emits a pulse having a finite pulse width, a number of transmissionphotons expected from the interrogation source 110 is able to becalculated. While photon flux (e.g., rate of photon reception) fromradiation source 401 may be steady over time, the photon flux frominterrogation source 110 is able to be much higher, depending on outputpower and duty cycle.

The time window within which a SPAD is active before generating a signalin response to background noise provides a useful time window for pixelactivation over one measurement period. Stated another way, an object tobe measured (e.g., object 440) will be within such a distance that theTOF of the interrogation beam 425 and returned illumination 430 will notexceed the time within which a SPAD will fire due to background noise.There is a therefore a maximum useful TOF, which corresponds to amaximum range of object distance measurement (light travels atapproximately 0.3 meters per nanosecond which, due to the out-and-backnature of the measurement, reduces a maximum range determined by afactor of 2). In measurement scenarios where an approximate range to aninterrogation object is known, accumulation of ambient photons (e.g.,background noise) may be minimized by coordinating activation of theimage sensor 115 with the expected combined TOF of the interrogationbeam 425 and returned illumination 430, which is based on the knownapproximate range to the interrogation object. Stated differently,activation of photodiodes of the image sensor 115 may be TOF-gated, inorder to reject ambient photons from ranges in the depth of view outsidethe expected range to an interrogation object. This TOF need not bedirectly measured by TOF-specific circuitry, but rather is able to bedetermined based on distance measurement(s) according to triangulationas described herein.

The pulse window and the pixel window define one distance measurementperiod. In this manner, interrogation beam angle 411 can be correlatedto returned angle 416, which, along with distance 120, provide distancedetermination to object 440. It is important that received photons atthe image sensor 115 are unambiguous, such that photons received atangle 416 are properly attributed to the interrogation source angle 411from which the photons were emitted. Otherwise, desynchronization mayoccur between the interrogation beam angle 411 and the receive angle416, resulting in erroneous distance calculation to object 440. However,the time available for each pixel must be limited by photon budget anddistance, as well as total time allowed for each sweep of theinterrogation source (e.g., 110) or the image sensor 115 will be subjectto firing due to noise, as well as desynchronization of the transmissionand reception of the interrogation beam.

As may be appreciated, the function and timing of the image sensor is ofparticular importance for an optical interrogation system according tothe present disclosure. The concept of determining a distance to anobject based upon triangulation can be further enhanced by thearchitecture of the sensor, in order to minimize power consumed by thesensor and the spurious information that an array of highly sensitiveavalanche photodiodes might generate due to noise. For example, if awhole array of an image sensor is configured in Geiger mode and enabled,a large portion of the SPADs would fire nearly simultaneously, butperhaps only one (or so) would be due to the interrogation source 110(e.g., the transmission signal), dependent on the exposure time.Conversely, selective activation of SPADs within the array is able toreduce array-based DCR noise for the image sensor 115 area of interest,and thereby increase the SNR of the optical interrogation device.

Programmable Image Sensor Row & Column Shutter

FIG. 5 is an illustration of an exemplary image sensor architecture 500,in accordance with embodiments of the present disclosure. The imagesensor architecture 500 includes an array of photodiodes 140, start,setup, ranging control inputs 505, high voltage and supply circuits 510,multiplexer banks 515, sense amplifiers 520, detect and latch circuitry525, multiplexer sequencer 530, address latches 535, and address andedge trigger input/output circuitry 540. The architecture 500 furtherincludes a subset of pixels 550 of the array of pixels in the imagesensor, referred to herein as a “sub-array” or “subset of the array” ofpixels, the whole array having size R rows and C columns.

With the use of a highly sensitive photodetector in an image sensor(e.g., image sensor 115 of FIG. 1), a low power, high resolution opticalinterrogation system is able to determine range to an object. Typically,with a small cell size and at IR wavelengths, a SPAD pixel 140 requiresreceipt of approximately 20 photons in order to fire. In an embodiment,accurate position trigonometry is determined by reporting only theaddress of a pixel that develops a signal, within a particular timeframe (e.g., pixel time frame), based on receipt of photons from anincident interrogation beam reflected from an object of interest. Sincethe optical interrogation system controls the timing of interrogationsource activation (the device controls at what time an outgoing signalis generated), the device is able to be programmed to anticipateapproximately over what time period a return signal should arrive (forobjects present in a known range, e.g., within a known depth of field).

In order to operate an optical interrogation system at low power, powersavings should be realized in as many components as possible (whilestill maintaining a suitable resolution capability). While the use ofhighly sensitive SPAD-based pixels reduces the power requirement of theinterrogation source, still further gains are able to be made withcareful consideration of the manner in which individual SPADs areenabled within the image sensor array. As discussed herein, an avalanchephotodiode requires a large reverse bias in order to enter Geiger mode,the initiation of which (via, for example, the high voltage and supplycircuits 510) requires a certain power for each individual diode.Further, an avalanche diode in Geiger mode (e.g., a SPAD) is not able tomaintain this state indefinitely, but rather will spontaneously firewith elapse of a relatively short time (depending on conditions such asambient light, cell size, temperature, overvoltage of the diode).Therefore, it may be undesirable to activate all photodiodes in theimage sensor array simultaneously, as the likelihood increases of firingevents arising from a source other than the interrogation source, asdoes the power consumed by enablement of photodiodes.

An approach to reduce both power consumed and the probability of pixelactivation due to a non-interrogation source event is to activate only asubset of photodiodes in the image sensor during a measurement period.Stated another way, a “page” (e.g., a sub-array 550 of pixels) isactivated and enabled to detect an interrogation source transmissionduring a given time period (e.g., a pixel activation time), and togenerate a signal, while other pixels in the image sensor array remaindormant. Activation of a reduced number of photodiodes reduces powerconsumption, and also reduces the likelihood of spurious spontaneousactivation due to background noise (e.g., by removing DCR, ambient lighttriggering from non-enabled pixels in the array).

For a field of view where no prior knowledge of the location/distance ofobjects exists, all pixels in the image sensor are optimally enabled inorder to provide the greatest opportunity to detect reflectedillumination from an object in the field of view. Interrogation of afield of view with some knowledge of prior location/distance ofobject(s) therein is performed using a sub-array of pixels in the imagesensor. A sub-array of pixels can be used because knowledge of expectedlocation/distance to an object in the field of view provides an expected(approximate) location of reflected illumination from that object, at agiven interrogation beam angle from the interrogation system. Such ascenario presents a use of the interrogation system's capability ofconfiguring the image sensor to activate only a subset of the pixelarray. In conjunction with the optical interrogation system, this may inturn increase the frame rate and reduce power consumption of theinterrogation system. This scenario may exist during, for example, anupdate of a known scene in the field of view (e.g., a field of view thathad been measured recently).

The size of the sub-array 550 can be based on a prediction of wherephotons should arrive, given a knowledge of the volumetric space beingmeasured (e.g., the minimum and maximum distance). The size of thesub-array 550 may need to increase or decrease, due to factors such asthe position of objects within the field of view, the power of theinterrogation source, the ambient light level, the overvoltage level ofindividual SPADs, etc. The specific subset of photodiodes active for aparticular measurement period is able to change from measurement tomeasurement (e.g., at the interrogation source is moved to sampledifferent portions of the field of view), thereby generating a “rollingsub-array” of active image sensor photodiodes 550. This is depicted inFIG. 5 by several sets of pixel sub-arrays 550, moving along the sensorarray over time as indicated by the dashed arrow. As a non-limitingexample, a sub-array 550 is configured using 3 adjacent rows of theimage sensor array, and 4 adjacent columns (a 3×4 sub-array size). Asanother non-limiting example, an array-high sub-array 550′ is configuredwith all rows and several (e.g., 3) adjacent columns active. Sub-array550′ may be used, for example, with interrogation beam 125 configured infan (e.g., line) mode.

In one aspect, this rolling sub-array 550 may be considered a kind ofrolling shutter for the optical interrogation system. The movement ofthe sub-array 550 is typically incremented at the same rate that a pixelis activated/deactivated for a measurement period—also termed the “pixelrate.” As described herein, the pixel rate is determined by factors suchas the angular rate (e.g., resonance frequency of a MEMs based scannermirror) at which an interrogation source moves within the measurementfield of view, a mode of the interrogation source (e.g., stepped mode or“fan” mode), the ambient light level, etc. Furthermore, by reporting apixel event via pixel address only, high throughput of information isable to be attained.

The timing of the pixel sub-array 550 activation with respect to theactivation of the optical interrogation source is able to be institutedin several ways. In a synchronous mode, the time between successivepulses of the interrogation source is determined at the opticalinterrogation system level, by such factors as the ambient light level,the power of the interrogation source, the pulse time, etc., and thesub-array 550 is activated at the same time that a pulse from theinterrogation source is fired. Stated differently, the interrogationsource and the pixel sub-array 550 are controlled in a synchronized,pulse-by-pulse fashion. Constraints on the period between pulses includethe number of photons required to activate an individual photodiode inthe subset of pixel sub-array 550, as well as the maximum TOF fordetermining an object's position (which dictates a maximum range for thedetected object). In an embodiment, each pixel completes a sequence ofreaching excess reverse bias threshold, maintaining threshold for up tothe maximum TOF, firing (if a photon is detected), and quenching, withinthe time period that one interrogation source pulse is enabled. If themaximum TOF is greater than the pulse period of the interrogationsource, then multiple pulses of the interrogation source will be presentduring the same measurement period of the pixel sub-array 550, leadingto distance determination ambiguity. A trigonometric determination ofdistance requires a unique interrogation source angle, along with aunique receive pixel address. Therefore, minimizing the time requiredfor the SPAD to complete an enablement/firing/reset cycle is beneficialto the operation of the optical interrogation system, as it enables agreater frequency of interrogation source pulses, along with a finerimage sensor array resolution. One manner of reducing a SPAD operationcycle is by minimizing the quenching time of the SPAD. This is able tobe accomplished via active quenching of the SPAD, whereby an active loadis enabled at the time that onset of an avalanche current from the SPAD.In an embodiment, if the TOF exceeds (interrogation source frequency)⁻¹,both the interrogation source pulse and the pixel sub-array 550 areincremented and no event is recorded.

An alternative timing of pixel sub-array 550 is asynchronous operation,wherein the interrogation source and the pixel sub-array 550 aresimultaneously enabled, but the timing between interrogation sourcepulses is determined by the timing of detection events. In anembodiment, the interrogation source emits a pulse only when the pixelsub-array 550 receives a return signal (e.g., reflected photons), orwhen a system time-out occurs. A system time-out is configurable at thesystem level, and is able to be based upon ambient light level, maximumrange to an object, interrogation source pulse power and pulse width,etc. In this manner, a variable or self-timed operation of the opticalinterrogation system is able to be implemented. For objects located atlonger distance (e.g., several meters), the frame rate of the systemwill be variable. For near objects operation in an asynchronous modeenables a faster interrogation source pulse and pixel sub-array rate, asmaximum TOF timeout is not a factor when near objects are present formeasurement.

In an embodiment according to the present disclosure, multiplexer banks515 are coupled to the rows and columns of the image sensor array. Inone embodiment, multiplexer banks 515 select outputs according to thecurrent sub-array activated, for example, selecting 3 rows out of 768,8-bit resolution, and 4 columns out of 1024, 8-bit resolution,corresponding to a current sub-array. Other multiplexer bankconfigurations are possible. The outputs of the multiplexer banks 515connect to row- and column-sense amplifiers (or comparators) 520,respectively. In an embodiment, the first photodiode to develop asignal, in any of the rows, will generate a “winner-take-all” detect andlatch 525—for example, of 3 active rows, 1 will be output as thedetection signal. Likewise, the column corresponding to the signalingphotodiode is able to generate a detect and latch 525 output (e.g., 1 of4). According to embodiments of the present disclosure, the outputsignal reports only fired pixel locations (e.g., only the addresses ofthe pixels from the “winner-take-all” detect and latch), and does notinclude information from other pixels. Other combinations of detect andlatch 525 are possible, and are consistent with the spirit and scope ofthe present disclosure. The combination of the first row address to beactivated, and the first column address to be activated, gives a uniquepixel address associated with the interrogation source beam enabledduring the measurement period. This pixel address comprises the outputsignal, corresponding to the particular interrogation source angle. Forexample, the output of the image sensor is 2 bits row information and 2bits column information, since ranging information is based on adetermined sub-array address—therefore, signal management andinput/output challenges are able to be significantly reduced compared toconventional ranging systems.

SPAD timing is able to be externally gated in order to reduce noise,although at an expense of system measurement range (e.g., maximum depthof field). For reduced chip size, external timing and controlelectronics may be used—there is however a tradeoff between design of aprinted circuit board (PCB) versus a chip design level. Control inputsignals are able to be used to generate self-timing, via edgetriggering, of programmable setup, pixel enable, and multiplexersequencing 530. The multiplexer sequencer 530 includes timing andcontrol logic that is triggered from a start signal 505.

During the measurement phase of the optical interrogation system, as theinterrogation source is incrementing an angle of transmission of theinterrogation beam, the presence of two objects at differing distancesfrom the interrogation system may give rise to a situation wherein asudden apparent change in object position develops, or alternatively,when an object's presence is not properly detected. FIGS. 6 and 7illustrate embodiments of one such scenario, wherein a near objectobscures at least a portion of a far object.

FIG. 6 illustrates an environment 600, wherein a near object 610 and afar object 615 are measured by optical interrogation device 105. At afirst measurement period, an interrogation beam 60 is emitted, whichreflects off of near object 610 and is incident on the image sensor atangle θ₁. At a second measurement period following the first measurementperiod, a interrogation beam 62 is emitted, which is reflected off offar object 615 at angle θ₂. Because the objects 610 and 615 are adifferent distances, the return angles θ₁ and θ₂ differ, to an extentthat depends on the relative distance between the two objects 610 and615. The implication of these differing return angles, over the courseof just one measurement period, is that the returned illumination fromthe second measurement will be received at an array address severalpositions removed from the address of the first received signal, leadingto an apparent shift in the receive angle, proportional to the distancebetween objects. This skip angle is denoted θ_(s) in FIG. 6. Thisscenario causes a step function in the position of the object returnedby device 105, owing to the distance being determined usingtriangulation. In general, a region 650 exists wherein objects fartherfrom interrogation device 105 than a near object will not be imaged.

FIG. 7 also depicts an environment 700 having multiple objects formeasurement, near object 710 and far object 715. The geometry of theobjects 710 and 715 is such that, over a certain range of interrogationbeam angles, reflected illumination resulting from incident illuminationon far object 715 will be blocked from reception at the device 105 imagesensor by near object 710. This situation may be referred to generallyas object occlusion. In the example, a first interrogation beam 70 isincident on far object 715, and, narrowly missing object 710, formsreflected beam 72 and is incident on the image sensor of device 105. Thefollowing interrogation beam 71 is incident on object 715, but thereflection of 71 is blocked from the view of the image sensor by object710. This causes failure of the device 105 to measure the distance tothose portions of the far object 715 occluded by near object 710. At alater point in the scanning cycle, an interrogation beam 78 is emittedthat is incident upon the near object 710, and reflected illumination isreceived again at the image sensor of interrogation device 105. Ingeneral, a region 750 exists wherein objects farther from interrogationdevice 105 than the object 710 will not be imaged, due to occlusion ofthe returned illumination.

An approach to account for these sudden jumps in interrogation beamreturn angle (e.g., of FIGS. 6 and 7) is to enable a larger group ofpixels at one time (e.g., to increase a size of a sub-array or page ofpixels, e.g., sub-array 550 of FIG. 5). At a greatest extent, this sizemay be up to half the field of view (in the same dimension of theinterrogation beam scan, e.g., along the columns). In this case, thewidth of the sub-array of pixels may be up to half of the columns.Further, the sub-array of pixels may require some depth in the scandirection orthogonal (e.g., the rows) in order to account formechanical/optical misalignment between the controlled and actualposition of the interrogation source compared to the image sensor,and/or movement of the optical interrogation system with respect to thefield of view. Sources of mechanical/optical misalignment in theinterrogation source position include MEMs steering and responselinearity. These are able to be accounted for by an increase sub-arraysize of active pixels. The distance determination error induced bymisalignment is related to the resolution of the pixel array—a finelyresolved array (small pixel size) will have greater returned positionerror for a given angular misalignment, because a greater number ofpixels will be skipped over. While this approach may cause somewhatgreater circuit complexity and power consumption for the opticalinterrogation system, a balance should be sought between the likelihoodof missing a returning set of photons from the interrogation beam due toa too small sub-array size (of sampling), against increasing backgroundnoise by observing (e.g., enabling pixels over) too large of an area.

The distance between the near and far objects (e.g., object 610 and 615)affects a skip angle—a smaller inter-object distance gives a smallerskip angle, which in turn leads to a reduced sub-array size requirement.In an embodiment according to the present disclosure, a sub-array sizeadjustment is made automatically, based upon a detected inter-objectdistance in a field of view of the optical interrogation device (e.g.,via a determined skip angle). According to an embodiment, the sub-array550 size is dynamically adjusted, accounting for changes in a skip angledue to for example, moving objects in the field of view, or to newlyintroduced objects.

The capability of the optical interrogation device to adjust theactivation of subsets of pixels in the sensor array (e.g., pixelsub-array 550 size) provides further functionality. In embodiments ofthe present disclosure, a frame rate with which the opticalinterrogation device is outputting frames is adjusted by reducing thenumber of rows that are activated in the image sensor array during(e.g., a frame zoom). Stated differently, the frame size is decreased byconsidering fewer than the total possible array rows (e.g., activatingfewer than 768 rows in image sensor architecture 500), which leads to aframe rate increase. The frame rate increase will be proportional to thereduction in rows scanned (as the scan time for the columns ismaintained, due to the resonant motion of the interrogation source). Thefull frame is able to be generated by a full scan of the interrogationsource, for example, an interrogation source resonant in parallel withan image sensor rows axis and stepped in a parallel axis to image sensorcolumns. The pulses of the interrogation source pulse activation areable to be limited to those regions contained within the reduced frameregion, and likewise a rolling pixel sub-array 550 is able to be activein only the reduced frame region. In an embodiment, the interrogationsource is activated in a pulse spot mode, and the stepped motion of theinterrogation source is reduced to cover only the rows in the frame zoomregion. Operation of the interrogation source and the image sensor inthis manner can lead to a reduction in the power consumption of both theinterrogation source and the image sensor array. Further, as notedabove, the frame rate may be increased, as a particular region in thefield of view is zoomed in upon.

A variety of implementations for activating a subset of image sensorphotodiodes are possible. As an exemplary alternative to a rollingsub-array of adjacent pixels, a coarse first scan is performed withactivation of pixels 8 columns apart, followed by a finer scan with 4columns apart, and then 2, and so on. In this manner, a coarse estimateof positions of objects disposed within the field of view isascertained. Refinement of individual positions is able to be made bygoing coarse-to-fine, thereby enabling fine-tuning of object positions,and also of which interrogation source angles produce “skip” angles. Inan embodiment, a calibration mode includes a coarse scan, with largecolumn-gaps of pixels that are active, with subsequent fine-tuning ofthe column spacing (e.g., number of columns in the column-gaps) and/orthe region of scanning (e.g., the rolling sub-array location within theimage sensor array). In such a manner, background noise is able to bereduced by setting the logic pulses more accurately with respect to theobjects locations in the present field of view—e.g., the logic pulsesthat enable the pixels on the determined columns to form the reducedrolling sub-array.

In an embodiment, a first scan is made to produce a first frame atmaximum resolution, with the location of detected receive signalsdetermining where initial objects are located. Following this initialhigh frame rate scan, changes are made to which pixels (columns) areactive, to exposure delay, to timing, etc. According to embodiments ofthe present disclosure, these changes are implemented by iterativeloops. As a non-limiting example, a determination is made of at whatpoint a signal detection stops (e.g., by continuing to reduce exposure).From this point, the exposure is increased to a point where a signalresumes, which is then correlated to a substantially optimized solutionfor imaging the object(s) in the present conditions. In this manner, asubstantial reduction in background noise is realized, along with anincrease in optical interrogation range. Such adjustments areimplemented at the system level.

A further technique for increasing frame rate of the image sensorincludes an all-row scan mode (e.g., an array-high, swipe ofinterrogation source light “fan”). In this mode, the pixel page (e.g.,sub-array 550) can be an array-high page, with a determined subset ofactive columns that increment with interrogation source pulses. Framerate may be substantially increased, with increased system bandwidthrequirements, as all of the pixels will be activated with each angularsweep of the interrogation source.

Ranging Via Integrated Time-of-Flight

In an embodiment of the present invention, a single and/or few photondetection array is able to be configured with time-encoded photon firinginformation per pixel (e.g., pixel 140 b), coupled with synchronizationinterfaces between an interrogation source (i.e. light source) and thedetector, in order to allow TOF and ranging information to be obtained.As an example, to achieve 1 cm of free space resolution requires eventresolution (i.e. photon travel time from the light source of theinterrogation system to the target scene and back to the image sensor ofthe interrogation system) with an accuracy of approximately 30picoseconds. The approach described herein is able to be implemented toevolve 3D imaging and ranging systems from high power broadcast orscanning patterns to low power photon bursts (i.e. flashed or pulseddirected laser beams) with highly sensitive photon detector arrays thatare designed to yield x-, y-, and z-coordinate information and eventtime, thus achieving an essentially digital photon detector that reducespost-processing requirements of image data.

As described herein, a SPAD pixel array (e.g., array of photodiodes 140)may be used in combination with a pulsed and/or steered interrogationsource (e.g. a laser light source) for 3D imaging and rangingapplications. In an embodiment, the interrogation source and the imagesensor array are coaxially located through the same optical aperture.Careful alignment should be made of the image sensor and interrogationsource, as the highly sensitive SPAD devices will respond to ambientlight as well as to the interrogation source. In an embodiment of thepresent invention, an optical interrogation system comprises a lightsource operatively coupled to a sensing element. As an example, thelight source is a vertical cavity surface emitting laser (VCSEL) and thesensing element may be a SPAD-based pixel contained within an array ofphotonic pixels. In such a system, the time delay between emission ofthe light source and detection by a pixel is able to be monitored, thetime delay reflecting travel time of at least one photon from the lightsource to a portion of the scene and back to the sensing element, thetravel time corresponding to the distance between the system and aportion of a scene. TOF computation and subsequent readout is able to beintegrated into SPAD pixel functionality with the SPAD pixel capable ofturning off a charge integrator in such a manner that the pixel storesTOF information (as opposed to, e.g., storage of optical strengthinformation).

Monitoring of the time delay may be performed through different schemes,including, but not limited to, adding charge storage capability to thepixel sensor cell (i.e. control of the charge level of a capacitor).Capacitor charge level is able to then be correlated to time delay, thusproviding an estimate of distance from system to scene. In one example,an emission event of the light source is able to trigger discharge of acapacitor and a pixel detection event is able to terminate discharge ofthe same capacitor. In another example, an emission event of the lightsource is able to trigger charging of a capacitor and a detection eventis able to terminate charging of the same capacitor. In a furtherexample, the laser turn-on edge corresponding to emission of a lightpulse from the VCSEL is able to be used to initiate time measurementactivity (e.g. charging or discharging of a capacitor). Upon detectionof a return photon by the SPAD pixel corresponding to the light sourceemission, a leading edge of the avalanche current rise is able to beused to trigger an end to the time measurement activity (i.e. chargelevel change of a capacitor is ceased, thus resulting in a capacitorwhose charge is proportional to the time period associated with photontravel). In the case of capacitive charge used as a time measurementmeans, across the SPAD pixel array, each pixel is able to contain somedistance-related amount of charge in the respective storage capacitor,which corresponds to a particular portion of the scene and which is ableto be read-out and processed to provide spatio-temporal information ofthe scene as a whole. Such SPAD pixels are able to be read out in arrayform similar to techniques used for read out of conventional CMOS imagesensor (CIS) pixel arrays.

Such an embodiment provides a high frame rate, while costly MEMs-basedbeam steering, and other requirements may be circumvented. Further, ashortened and lower level light source illumination may be used due tothe heightened sensitivity of the SPAD pixel. It may be recognized thatin an embodiment the imaging system may be used with a synchronousand/or steerable photonic interrogation beam (e.g., light source) wherethe interrogation beam illumination provides a timing reference. Inaddition, because only a group of pixels within an array will betriggered, local decisions to trigger addressing events are able to beperformed. The present embodiment can be extended to the use of apredetermined interrogation pattern from a diffractive optical gratingin the interrogation beam with corresponding regions of imaging within aSPAD pixel array (e.g., structured light approaches). Embodiments asdescribed have lower light source power requirements and a capability ofproducing a direct image to, for example, a processor (e.g., processingunit 902). Thus, imaging sensor embodiments as described are well-suitedfor integration into a host device such as a mobile device with a hostprocessor.

The pixel array is able to further include a pixel scale microlens arrayto improve the fill factor of the pixel, that is, to focus incidentlight upon photosensitive region(s) of the pixel. The pixel array isable to contain sensitivity control (e.g., through reverse bias voltagelevel control) that can be implemented at the column level, row level,array region level, pixel level, global level, or any combinationthereof. Passive and/or active quenching, time gating, use of floatinggate programming (e.g. for mismatch calibration and/or pixel decisioncircuitry) are also able to be implemented.

According to embodiments of the present disclosure, an image sensor hasan array of photodiodes including a combination of TOF-enabled and nonTOF-enabled pixels (e.g., pixels 140 b and 140 a). As a non-limitingexample, a subset of TOF pixels 140 b are arranged in a coarse grid inan image sensor array, with remaining pixels comprising pixels 140 a.Such an arrangement can be used to simplify, or to augment, controlsystem logic for addressing pixels in the array (e.g., for generatingmoving sub-array 550). Using control of input signal from theinterrogation source, a coarse-grid calibration is able to be made usingTOF information from the TOF pixels 140 b, augmented by a highresolution scan using pixels 140 a of the image sensor array. A varietyof control schemes are possible, which are able to provide greaterprecision and/or quicker windowing for the image sensor array. Further,TOF pixels 140 b are able to be used, periodically, to assist in noiserejection for the optical interrogation device (e.g., by providing adirect TOF measurement to an object).

FIG. 8 is a flowchart 800 of a method determining a distance to anobject using an interrogation beam, in accordance with an embodiment ofthe present disclosure. Steps 801-809 describe exemplary stepscomprising the process depicted in flowchart 800 in accordance with thevarious embodiments herein described. In one embodiment, the flowchart800 is able to be implemented as computer-executable instructions storedin a computer-readable medium and performed by a computing deviceexecuting a process for determining a distance to an object.

At step 801 an object is interrogated with a photonic interrogationbeam. The interrogation beam is able to be for example interrogationbeam 125, from interrogation source 110 of optical interrogation device105. In an embodiment, interrogation source is a laser having a resonantMEMs scanning mirror (e.g., a vertical cavity surface-emitting laser(VCSEL). The interrogation source is able to emit radiation at an IRwavelength, or at other wavelengths. In an embodiment, the mirror isresonant in the x direction (e.g., along image sensor array columns),and stepped in the y-direction (e.g., incremented along image sensorarray rows). A resonant scanning mirror is able to be used to sweep aninterrogation beam 125 across a field of view for the interrogationdevice 105, and thereby interrogate an object in the field of view. Inan embodiment a laser operates in a “spot” mode; alternatively, a laserinterrogation source operates in a “fan” mode, emitting a broad stripeof illumination.

At step 803 an array of avalanche photodiodes (e.g., array of SPADphotodiodes) is activated, the avalanche photodiodes operating in Geigermode, the activation occurring substantially concurrent with activationof the photonic interrogation beam. In an embodiment, only a subset ofthe array is activated during a pulse of the interrogation source. In anembodiment, the subset of the array changes in a time-varying manner,and is incremented in concert with successive pulses of theinterrogation beam (e.g., rolling sub-array 550). The size of thesub-array is able to be altered, and can be based upon interrogationsource intensity and rate of movement, and the range to objects in thefield of view, among other factors.

At step 805 a reflected beam is received from the object (e.g., beam130, received at image sensor 115). An optical system (e.g., opticalsystem 135) may be included in the image sensor. The optical device isable to focus and/or filter incident reflection onto the image sensor.In an embodiment, deactivating the array occurs substantially concurrentwith reception of the reflected beam, following generating the signal atstep 807.

At step 807 a signal is generated, corresponding to a characteristic ofthe reflected beam. In an embodiment, generating the signal is based onthe receiving the reflected beam by a first array element comprised byan active subset of the plurality of subsets, and any detection of thereflected beam by a remainder of array elements in the array by theactive subset is not included in the signal. In an embodiment, eachphotodiode of the array of avalanche photodiodes includes at least onecharge storage element, and activating the array terminates a changingcharge level of the at least one charge storage element during a timeperiod between an emission from the interrogation beam and the receivingthe reflected beam from the object. In an embodiment, the storageelement is a capacitor, and the charge level is comprised by the signaland provides a measure of the time between an interrogation sourceemission and reception of reflected illumination (e.g., TOFinformation).

At step 809, a distance is determined to the object based upon thesignal. In an embodiment, the angle that the returned light comes intothe optics (e.g., angle 116) determines the column of the image sensorarray having photodiode activation (e.g., the position within the row).This pixel address, along with an optical interrogation source angle(e.g., 111), provides an estimate of returned light angle 116, andtogether with the distance between the interrogation source and theimage sensor, these provide a distance determination (viatriangulation). In an embodiment, a TOF computation and subsequentreadout is integrated into SPAD pixel functionality (e.g., pixel 150 b),and the SPAD pixel toggles operation of an integral charge integrator insuch a manner that the pixel stores TOF information.

Exemplary Ranging Device

An optical interrogation device according to the present disclosure mayinclude various configurations and may be used in a host electronicdevice employing an optical interrogation device. Such electronicdevices include but are not limited to wearable devices and otherportable and non-portable computing devices, such as glasses, watches,cellular phones, smart phones, tablets, and laptops. As presented inFIG. 9, an exemplary host device upon which embodiments of the presentinvention may be implemented includes a general purpose computing systemenvironment 900. The host device includes an optical interrogationsystem 907. According to an embodiment, the optical interrogation system907 comprises: an interrogation source on the system (e.g.,interrogation source 110) that illuminates a target scene (e.g., withlaser light) during image capture; a single and/or few photon pixelarray image sensor (e.g., image sensor 115) located on the system at acontrolled location (e.g., horizontally offset from the interrogationsource 110); and circuitry within the image sensor that converts anoptical image of a target scene into an electronic signal for imageprocessing. The optical interrogation system 907 optionally comprises animaging lens and/or microlens array, and/or a filter (e.g. an opticalwavelength bandpass filter) to reject ambient light during imagecapture.

In its most basic configuration, the computing system 900 may include atleast one processor 902 and at least one memory 904. The processor 902generally represents any type or form of processing unit capable ofprocessing data or interpreting and executing instructions. In certainembodiments, the processor 902 may receive instructions from a softwareapplication or module. These instructions may cause the processor 902 toperform the functions of one or more of the example embodimentsdescribed and/or illustrated herein. According to embodiments of thepresent disclosure, the processor 902 is configured to receive images ofa target scene from the image sensor, and to estimate distanceinformation of the target scene from the image.

The memory 904 generally represents any type or form of volatile ornon-volatile storage device or medium capable of storing data and/orother computer-readable instructions. In certain embodiments thecomputing system 900 may include both a volatile memory unit (such as,for example, the memory 904) and a non-volatile storage device 908. Thecomputing system 900 also includes a display device 906 that isoperatively coupled to the processor 902. The display device 906 isgenerally configured to display a graphical user interface (GUI) thatprovides an easy to use interface between a user and the computingsystem.

In an embodiment, computer system 900 comprises instructions forperforming processes according to aspects of the present disclosure,where the instructions may be stored on memory 904, or storage 908. Forexample, the computer system 900 may comprise object distancedetermination instructions 913, where object distance determinationinstructions 913 contain instructions causing computer system 900 toperform a process of determining a distance to an object according toembodiments of the present disclosure (e.g., process 800).

As illustrated in FIG. 9, the computing system 900 may also include atleast one input/output (I/O) device 910. The I/O device 910 generallyrepresents any type or form of input device capable ofproviding/receiving input or output, either computer- orhuman-generated, to/from the computing system 900. Examples of an I/Odevice 910 include, without limitation, a pointing or cursor controldevice (e.g., a touch-sensitive device and/or mouse), a speechrecognition device, a keyboard, or any other input device.

The communication interface 922 of FIG. 9 broadly represents any type orform of communication device or adapter capable of facilitatingcommunication between the example computing system 900 and one or moreadditional devices. For example, the communication interface 922 mayfacilitate communication between the computing system 900 and a privateor public network including additional host devices and/or computingsystems. Examples of a communication interface 922 include, withoutlimitation, a wireless network interface (such as a wireless networkinterface card, and/or a Bluetooth adapter), a wired network interface(such as a network interface card), a modem, and any other suitableinterface. In one embodiment, the communication interface 922 provides adirect connection to a remote server via a direct link to a network,such as a cellular network and/or the Internet. The communicationinterface 922 may also indirectly provide such a connection through anyother suitable connection. The communication interface 922 may alsorepresent a host adapter configured to facilitate communication betweenthe computing system 900 and one or more additional network or storagedevices via an external bus or communications channel.

Pixel Circuitry

FIG. 10 depicts a block diagram of exemplary pixel circuitry 1000 inaccordance with embodiments of the present invention. The exemplarypixel circuitry 1000 includes a photodiode 145, a latch circuit 1005,and decode and control circuitry 1010. The photodiode 145 generates acurrent pulse upon receipt of illumination, which can be translated intoa voltage pulse by an electrically coupled resistor (e.g., 220 of FIG.2). The voltage pulse may be used directly for logic by the pixelcircuitry 1000, for example enabling direct utilization of a photodiodeactivation event (e.g., firing) to set a latch at the latch circuitry1005.

FIG. 11 depicts an exemplary transistor level schematic pixel circuit1100 according to an embodiment of the present disclosure. The pixelcircuit 1100 includes buffering elements 1125 (e.g., corresponding tobuffer 225 of FIG. 2), latch elements 1105 (e.g., corresponding to latch1005), and decode and control elements 1110 (e.g., corresponding todecode and control 1010). The pixel circuit of FIG. 11 is gated bycolumn and row enable signals (ColHVen and RowEn), and has aground-referenced photodiode. In an embodiment, a photodiode isconfigured to operate in Geiger mode (e.g., operate as a SPAD). Theground-referenced photodiode, upon receipt of sufficient photonillumination (e.g., several photons) generates a rapid current pulsethat is translated, via a resistor, into a voltage pulse. The voltagepulse may be used directly for logic by the pixel circuitry, therebyenabling direct utilization of a photodiode activation event (e.g.,firing) to control functionality of the image sensor. For example, thevoltage pulse may be a positive pulse (relative to ground) that is usedas input for a transistor. According to embodiments of the presentdisclosure, the voltage pulse generated from the photodiode event isable to develop a logic level voltage which may be, for example, inputdirectly into CMOS logic in order to set (or reset) the set-reset latchcircuit of elements 1105.

According to embodiments of the present disclosure an image sensor(e.g., image sensor 115) comprises an array of pixel circuits 1100, anda set value for the latch 1105 is able to cause the image sensor arrayto report the address of the fired pixel, which can be used to determinea return angle of an interrogation beam reflected from an object. Asdescribed herein, with a determined return angle (e.g., angle 116 ofFIG. 1) distance to an object via is able to be found via triangulation.As a non-limiting example, a positive voltage spike can be inverted by asource follower to provide a negative voltage spike relative to Vcc,which can be a set signal to a set-reset latch. The pixel architectureaccording to FIG. 11 does not require high gain analog sense amplifiersor analog time-to-digital converters, which enables a decrease in bothpower requirements and measurement latency for the system due to thelack of analog components. The pixel architecture 1100 according to FIG.11 is a completely digital pixel.

According to embodiments of the present disclosure, the photodiode isactively quenched in order to control the time period between potentialsuccessive firings of the photodiode (e.g., in order to minimize thereset time of a SPAD after a firing event). Conversely, in many otherapproaches the position of a resistor is at the top of the photodiode,and the photodiode is anchored at ground. Such an architecture resultsin a negative pulse when a SPAD fires from a photon detection event,which is less useful for a system that is designed to directly utilize aphotodiode activation event as a direct input for CMOS logic.Conventionally, a SPAD may be AC-coupled (e.g., a photodiode coupledwith a capacitor) to perform level shifting in a passive circuit. Sucharchitectures are substantially slower than those of the presentdisclosure, and do not allow active quenching or gating of a pixel.Further, sense amplifiers used to sense a small current are not requiredin architectures according to the present disclosure.

According to an embodiment of the present disclosure the pixel circuit1100 is gated (e.g., the photodiode is selectively activated). In aquiescent state, the photodiode is ungrounded and there is no voltageacross the photodiode, and thus the photodiode is unable to fire even inthe event of incident photons. According to embodiments herein, thephotodiode is powered separately from logic circuitry (such as thelatch) by a high voltage rail. High voltage components of the pixelcircuit 1100 are limited to those elements within box 1145, namely, thehigh voltage supply line, the photodiode cathode, and junction A (e.g.,interconnection with the anode of the photodiode). When the pixel isenabled—by control lines (e.g., ColHVen and RowEn) controlled at thesystem level—photons incident at the photodiode may generate a signal(e.g., a current pulse from an avalanche breakdown event) that leads toa voltage pulse. As shown in FIG. 11, a firing pulse may be generated ata node B only when FireEn, ColHVen, and RowEn are at a high level. Thisarchitecture provides complete pixel gating control, and thereforecontrol over the set of pixels in the image sensor array that areenabled for each particular measurement period (e.g., specific,general-purpose pixel control for variable pixel page size). In anembodiment a voltage clamp is used in buffering elements 1125 to limitthe voltage spike at the gate during a firing event of the photodiode.Further, through limiting the voltage spike, the voltage clamp serves tomaintain a voltage level of the photodiode that is closer to the targetphotodiode bias level, and thereby reduces the time required to resetthe photodiode to a state of readiness for a subsequent firing event.

In this manner the pixel is gated, and the (enabled) ground-referencephotodiode is able to set a latch upon a firing event. According to anembodiment, RowEn allows HV image sensor column control in an HV mode,and column access in Read mode. HV bias across the diode may beaddressable by row and column, while a READ signal prevents columncontention in HV mode. According to an embodiment, read is addressableby ROW, and columns are sampled for firing event information from theimage sensor array photodiodes. Further control logic enables a globalreset of the latch (e.g., for a later ranging measurement)—a latch resetsignal LtchRst is enabled by a low pulse, and is applied to the activerow(s) in order to reset the photodiode(s).

The pixel architecture as presented in FIG. 11 enables a fast, powerefficient, serial self-sequencing readout of fired pixels for an imagesensor architecture. According to an embodiment of the presentdisclosure, a pixel latches with a firing event responsive to receipt ofillumination reflected from an object that has been interrogated by aninterrogation source of a ranging system. This latch enables a readoutof the address of the pixel that has fired, which, along with knowledgeof the interrogation system (e.g., the angle of the interrogation beam,the distance between the interrogation source and the image sensor),provides information needed to determine a distance to an interrogatedobject.

Referring now to FIG. 12, an exemplary pixel circuit 1200 includingbuffer 1225 and decoder elements 1210 is depicted. The pixel circuit1200 can further include quench feedback circuitry 1230. According to anembodiment, the pixel circuit 1200 is operable to generate a columnaddress value when a pixel of image sensor array 115 is activated basedon received illumination. The column address from decode circuit 1210can be used to set a row latch at the end of the row in which the pixelresides. Further, activations of the pixel can forward a disable signalto other pixels (e.g., other pixel circuits 1200) on the same row (e.g.,those corresponding to other columns of the row), and in this mannersignal contention can be minimized. That is, a first column activated onthe row sets a latch, stored outside the image sensor array 115, andalso disables other pixels of the row. Pixel circuit 1200 is well suitedto image sensor architectures interrogating in a fan mode (e.g., linescan mode), for example, the image sensor architecture described forFIG. 16.

FIG. 13 depicts an embodiment of circuit logic for image sensor columnself-timing enablement, as well as readout ripple-through functionality,based upon a pixel architecture such as that described for FIG. 11.According to embodiments of the present disclosure, circuit logic isable to receive a signal directly from the photodiode (e.g., a SPADfiring event), and thereby to (rapidly) set a latch. A first firingevent at a pixel in the image sensor is able to cause other pixels todeactivate. As a non-limiting example, once one latch has been set on arow in the image sensor, all other pixels on the row are disabled via aself-timing loop. This architecture enables a precise determination ofthe location of incident photons at the image sensor array. With thisinformation a distance determination may be made, as described viaembodiments herein. While a potential exists for adjacent pixels todevelop a signal (e.g., for adjacent photodiodes to fire), thispotential may be mediated substantially by careful control of timing forillumination from the interrogation source as described herein, as wellas, or along with, image sensor activation timing (e.g., pixel sub-arraygating).

In an embodiment, SPAD-based pixels contain a self-timed latch that isused to drive readout circuitry, reduce power consumption, and preventan event pile-up mode that is associated with using single photondetectors in high photon count environments. System level timingcommands (e.g., column- and row-enable signals) may be used to triggerself-timed asynchronous logic, at the pixel level. According toembodiments of the present disclosure, the system level timingsynchronizes activation of an interrogation source with activation ofone or more pixels in the image sensor (e.g., a pixel sub-array, whichis able to vary in size from one pixel up to the full image sensorarray). Once a column enable is set, a timing loop is triggered for apixel through a set-reset latch. If no photodiode activation isdetected, the latch is not set, and the next enable path is activated tocontinue the image sensor readout at the next pixel on the enabled row(e.g., the pixel at the adjacent column). The next column is preventedfrom being read until the current pixel has completed its readout cycle.Pixels report sequentially in the image sensor with a self-timed readoutperiod. In particular, only those pixels that experience an activationevent (e.g., a SPAD firing), and therefore set a latch, are the pixelsthat report for a given associated interrogation pulse. Stated anotherway, the only readout report from the image array is the address of thefirst pixel to set a latch, and further pixel reporting is prevented bynot enabling the sequential pixels during readout (e.g., a readout of afollowing pixel is enabled only if the present pixel does not set alatch). Unset pixel addresses are therefore sequentially skipped duringreadout by asynchronous feed-through logic (e.g., ripple-through), withreadout occurring more quickly and with lower data bandwidthrequirements than conventional systems, as only the address of the firstpixel fired on a particular row (or column, for column-by-column readoutconfiguration) is reported.

FIG. 14 depicts a block diagram 1400 of connected read logic circuitryaccording to an embodiment of the present disclosure (e.g., side-by-sidelogic circuitry, with global timing circuitry). The logic circuitryincludes column bypass logic along with an external timing pulsegenerator for column enablement. The timing is configured such that areadout of a current column state may be made, with a next columnenablement and readout according to similar logic. The particular timingis configured at the system level, and may be adapted according toimaging conditions (e.g., ambient light level, interrogation sourcepower, estimated distance to an imaged object, etc.). According toembodiments of the present disclosure, all columns are input when aphotodiode Read Row is set. In one aspect, ripple-through logic isenabled when a column has a low value (e.g., a photodiode is off). Asthe readout progresses sequentially through the array, the first columnreturning a high value halts the ripple-through logic, and a value isable to be set corresponding to the high value of the column. In anembodiment, the column high value sets a ROM row to a high value, untila timer resets and a Next Enable is set. This process is repeated in thenext (e.g., sequential) column, until the final column in the array isreached. Those ROM Rows having photodiodes that have been set (e.g.,from a column high value) are read out at the image sensor input/outputcircuitry. The readout ripple-through logic is able to be implemented inseveral image sensor embodiments, including the embodiments describedbelow for FIGS. 15 and 16, where the ripple-through logic is used duringreadout of ROM column address, or row column registers, respectively.

Image Sensor Architecture

FIG. 15 is a schematic of an image sensor architecture 1500 according toan embodiment of the present disclosure. The image sensor architecture1500 includes an array of photodiodes, a serial programmable interface(SPI), a column control mux, a row mux, input/output control and timingcircuitry, a fast ROM decoder, and configuration circuitry for row- andcolumn sequencers. Each pixel of the image sensor array is gated andself-latching, as described herein (e.g., FIGS. 10 and 11). In oneaspect, the image sensor is a self-reporting pixel sequencer, having apre-programmed sub-array size 550 (up to the full image sensor array).The pixel is digital in nature and has a value of either 1 or 0,corresponding to a latch that has or has not been set, respectively,during a distance measurement period. The image sensor array is dividedinto user-configurable pages, which are able to be incrementedsequentially as the angle of the interrogation beam is varied, and/orthe addresses of activated pixels are able to be coordinated with theinterrogation beam transmit location in 3D space. The latter allowssimultaneous read of an exposed sub-array of pixels while exposing ageometrically separated sub-array, which can increase frame rate andthroughput, and is able to be configured at the system level.

The control logic (e.g., FIGS. 13-14) at the front end of the ROMdecoder enables a pixel, when set by a photodiode activation, to driveonto the fast ROM decoder—all other pixels being off at that time.According to embodiments of the present disclosure an output of theimage sensor array is a row driver signal into a fast ROM decoder, thesignal corresponding to a column of the image sensor having a firingevent, the ROM decoder having, for example, a lookup table containingthe address of the activated column for image sensor locationdetermination. The current enabled row is known (as it is controlled bythe imaging system), and the column location determined by the ROMdecoder provides the unique address of the photodiode that has beenactivated. In this manner, the image sensor architecture 1500 latches anactivation event at the pixel, and decodes the location at the column(via the fast ROM decoder). All of the rows of the image sensor areinputs to the fast ROM decoder, where pixels include logic as describedaccording to embodiments herein. An image sensor architecture accordingto the present invention outputs only the array address of a pixel thatis activated during a measurement period (e.g., by a SPAD firing event),and all further unread pixels on the row are deactivated and skippedover, thereby minimizing both the opportunity for firing due to a noiseevent (e.g., exposure to ambient light) and the bandwidth requirement ofdata transmission.

High-level (e.g., system level) timing commands are used forsynchronizing illumination light pulses from the interrogation sourcewith the image sensor activation of pixels. The timing commands are usedto trigger self-timed readout in the image sensor, which drives thesequential reporting of only those pixels which have fired in theassociated light pulse period (e.g., the pixels operate as asynchronousstate machines). After all rows have been read, a global reset(controlled at the system level) resets all of the pixels in the imagesensor for the next interrogation beam pulse (e.g., next measurementperiod). The row read mechanism uses the aforementioned pass-throughlogic functionality, for each row, to report only activated pixels(rows). Alternatively, a register bank can be used to latch a row scan,with readout occurring at a subsequent row scan.

Referring now to FIG. 16, a block diagram of an image sensorarchitecture 1600 is depicted according to one embodiment of the presentdisclosure. As compared with the image sensor architecture 1500, thefast ROM decoder functionality of image sensor architecture 1600 isincluded at the individual pixel level, and made internal to the imagesensor array (e.g., pixel architecture of FIG. 12). However, theset-reset latch circuitry is located external to the array, such that alatch is performed at the row level and only one latch is set per row.The architecture 1600 includes column address latches (e.g., a registerbank), including first latches to latch a row scan, and second latchesthat can be set via a shift from the first latches. The second latchescan be read during a subsequent row scan. The image sensor thereforelatches at the row level, and decodes at the pixel (cell) level. Asdescribed herein, ripple-through logic is included in the image sensorcircuitry such that the image sensor array is read along the row latchesat the end of the rows (rather than the columns, as described in FIG.15).

Image sensor architecture 1600 may be configured for a line scaninterrogation mode, where all image sensor rows are active concurrently.Based on the time when the interrogation source is activated, one ormore row lines of the image sensor array are activated. According to theimage sensor architecture 1600, the first photodiode activated on a rowwill claim the address bus for that row, and therefore a unique imagesensor array position is able to be determined for the returnedillumination from the interrogation source. In an embodiment there areten column address lines per row (greater or fewer address lines beingpossible), a hardwired ROM address for each pixel, such that when apixel is activated by incoming photons a discrete (e.g., single) columnaddress signals on the address lines to set a corresponding register.This provides the image sensor the capability of identifying the columnaddress of the pixel that was activated last, if one was activatedduring the measurement period.

This circuitry configuration may be less space efficient than otherimage sensor architectures disclosed herein, however, improved sensorflexibility can be realized with a column address ROM embedded in eachrow. Image sensor architecture 1600 is able to employ a sub-arrayapproach (e.g., moving sub-array 550, 550′) to control and mediatebackground noise and other factors during a measurement period.According to an embodiment, full- or partial rows in the image sensormay be enabled by system control substantially simultaneously, foreither line scan or dot scan interrogation mode, with the image sensorcapable of reporting a first photodiode firing event in each row.

The image sensor as depicted in FIG. 16, having a latch at therow-level, enables parallel operation in the sense that a readout oflatched values (that is, row values) can be made simultaneously with anexposure of a next line in the image sensor array. According toembodiments of the present disclosure, the image sensor includes twosets of registers—a first register set for a latched column address (theset including 1 register per row), and a second register set to receivethe shift of the value from the first register set. In this manner, witha first measurement cycle, a value of a latched column address for agiven row can be written into the first register corresponding to thatrow. The value is shifted to the second register, enabling subsequent,simultaneous operation of a second measurement (and latch) with a firstread operation (from the first measurement cycle). In this manner thelatency of a system employing an image sensor according to the presentdisclosure may be substantially reduced.

An exemplary ranging scenario for an imaging device using image sensorarchitecture 1600 includes an interrogation source configured to emit avertical line of illumination (e.g., line scan mode). The interrogationbeam incident upon an object having 3D contours will have a returnedillumination that is no longer strictly vertical, the resulting varianceoccurring according to the particular contours of the 3D object. Thisreturned illumination will therefore have a non-uniform aspect (e.g.,not uniformly vertical), represented by the curved, dashed line of FIG.16. The image sensor architecture operates to set a row latch when afirst photodiode is activated (e.g., a SPAD firing event), and the imagesensor circuitry is configured such that the setting of the latchprevents other neighboring photodiodes from activating until a masterreset is performed. The master reset is activated to ready the imagesensor for a subsequent measurement period. According to embodiments ofthe present disclosure, decision logic is incorporated by the imagesensor array such that a first detected photodiode activation on a rowclaims the column address for that measurement period, for the entirerow. This decision logic is replicated on each row of the image sensor,such that a row-by-row set is achieved. It will be appreciated thatwhile decision logic is described on a row-by-row basis, the imagesensor architecture may also be configured for column-by-column logicand measurement.

Therefore, for a vertical interrogation beam, there is a contour line ofactivated photodiodes on the image sensor, one per row, which is able tobe read. During a readout of the image sensor, if there is a row whereinno photodiodes were activated the ripple-through logic causes readout ofthat row to be skipped and the subsequent row to be read. In this mannerthe image sensor is made more efficient in both readout time and powerrequired. The ripple-through logic will continue until the next rowhaving a latch set during the measurement period is met, at which pointa read will occur to output the address of the activated photodiode onthat row. The ripple-through logic is self-timed, commencing with asingle input trigger signal to the image sensor array.

In another embodiment of the present disclosure, image processing andpattern recognition paradigms different from those used withconventional CIS pixels are able to be utilized which are based on abinary pixel output in combination with local decision making madepossible by the high photon sensitivity. As a non-limiting example, inthe case of SPAD pixels utilizing high voltage pins for reverse biasing,implementation of a floating gate programming scheme for mismatchcalibration and pixel decision circuits is made. In a further example,each pixel is calibrated and digitally corrected for mismatch with aparallel memory array using the same addressing decoder scheme as thepixel array. The embodiment described here is able to be used with, forexample, a massively parallel asynchronous event bus when used inconjunction with an adequately high speed interconnect and othercomponents. The embodiment described here may also be implemented in away such that the photon input timing and geometry (e.g. spot, pattern,line, or flash) are controlled with the interrogation light source andthe pixel decision response is tailored according to the interrogationlight source type. In an embodiment, a SPAD pixel is implemented usingdifferent configurations and/or methods of operation, including, forexample, use of a current mirror (e.g. for constant current operationregardless of load conditions), a latching scheme (e.g. to initializeand terminate event driven activities), internally triggered avalancheevents (e.g. to initiate time measurement activities), etc.

Single or Stacked Wafer Image Sensor Fabrication

According to embodiments of the present disclosure, a semiconductordevice can be fabricated on one substrate (e.g., pixel 140 b of FIG. 1),or on stacked substrates having at least two layers with portionsconfigured to operate at different voltages (e.g., pixel 140 a of FIG.1). According to embodiments of the present disclosure, fabrication of asingle substrate semiconductor device includes formation of photodiodes(e.g., 145 b of FIG. 1), which can be ground-referenced photodiodes,along with circuit elements (e.g., 150 b of FIG. 1) sharing a samesubstrate as the photodiodes, the circuit elements providing the logicand control (e.g., latches, decode and control, read logic, enablesignals) as described in the various embodiments herein.

Referring now to FIG. 17, a portion of a semiconductor device 1700 isdepicted. The semiconductor device includes at least two layers, a firstlayer 1740 and a second layer 1750. The first layer 1740 includes afirst portion 1745, and the second layer 1750 includes a second portion1755. According to embodiments of the present disclosure, the firstportion 1745 is configured to operate at a first voltage, and the secondportion 1755 is configured to operate at a second voltage. According toan embodiment, the first voltage and the second voltage are different.According to an embodiment, the first voltage is higher than the secondvoltage. According to an embodiment, the first layer 1740 includesdiffusions forming a photodiode (e.g., 145 a of FIG. 1, an activephotodiode, or a SPAD), and the second layer 1750 includes circuitrycorresponding to logic and control (e.g., 150 a of FIG. 1, latches,decode and control, read logic, enable signals). As a non-limitingexample, second layer 1750 logic and control circuitry can providecircuit logic functionality depicted in the portions of FIG. 11 notincluded in high-voltage portion 1145. The second layer 1750 can includetransistors, resistors, capacitors, and other components appreciated byone of skill in the art to provide the logic and control functionalitydescribed herein. The first portion 1745 can include an array ofphotodiodes.

According to an embodiment of the present disclosure, an image sensorarray is fabricated having a stacked wafer design, where a top wafer(e.g., a wafer corresponding to a top side of an imaging sensor arrayexposed to incident illumination) is doped to form SPAD cells (e.g., isa SPAD wafer), and a bottom wafer is formed to have logic and controlcircuitry to provide the structure and functionality of imaging sensorembodiments as disclosed herein. Referring now to FIG. 18, a portion ofan imaging sensor 115 is depicted having a wafer 1840 including SPADcells with photosensitive regions 145, the wafer 1840 stacked over alogic and control circuitry wafer 1850. While the SPAD wafer 1840 andlogic and control circuitry wafer 1850 are depicted as separated, inoperation (that is, implemented in a ranging system) the SPAD wafer 1840and logic and control circuitry wafer 1850 can be two substrates bondedtogether (as indicated by dashed arrows), or can be of one substrate, asdescribed herein.

A stacked wafer orientation of a SPAD wafer 1840 over a logic andcontrol circuitry wafer 1850 enables reduced pixel pitch P (that is,closer SPAD cells) compared to implementations where each SPAD cellcontains control and logic circuitry, on the same wafer. For example,pixel pitch P can be 3-4 microns. According to an embodiment of thepresent disclosure, a SPAD wafer is a high-voltage (HV) diffusion wafer,while the logic and control circuitry wafer is low voltage (LV) wafer.An LV wafer enables finer features than an HV wafer, increasing thenumber of transistors that can be formed on the wafer for a given space.

Referring now to FIG. 19, processing steps 1905-1915 for fabrication ofan imaging sensor array 115 having a stacked wafer design are depictedaccording to embodiments of the present disclosure. The portion ofimaging sensor array 115 shown in FIG. 19 corresponds to section A-Afrom FIG. 18. At step 1905, logic and control circuitry wafer 1850 isshown, including depletion regions, interconnections 1920, and athrough-silicon via 1930. According to an embodiment, the logic andcontrol circuitry wafer 1850 is formed on a low-voltage CMOS wafer.

Also at step 1905 a SPAD wafer 1840 is shown, the SPAD wafer 1840including depleted p-n junctions. SPAD wafer 1840 can be a CMOS wafer,for example a high voltage CMOS wafer. The p diffusion can correspondwith a photosensitive region of a SPAD cell and can be configuredsurrounded by n-diffusions, which act as a barrier from activation ofthe SPAD diode by neighboring SPAD cells. According to an embodiment,the SPAD diodes can share an n-well. Neighboring SPAD cells of the SPADwafer 1840 can further be isolated by isolation trenches 1940. SPADwafer 1840 includes metal interconnections 1920. The SPAD wafer 1840 canbe built as a silicon-on-insulator (SOI) wafer. The insulator can be,for example, an oxide layer. At step 1905 SPAD wafer 1840 can furtherinclude a support wafer. The SPAD wafer 1840 and logic and controlcircuitry wafer 1850 are co-designed (e.g., positions of interconnects),and have a same die size. According to an embodiment, interconnections1920 are an array of cathode/anode pads. According to an embodiment,high voltage wiring can be limited to cathode wiring only at the SPADwafer 1840, removing isolation requirements on logic and control circuit1850 wafer. In this embodiment array interconnects 1920 are reduced tolow voltage anode only.

At step 1910 the SPAD wafer 1840 is mounted face-to-face over the logicand control circuitry wafer 1850 in a stacked orientation (e.g.,flip-chip mounted), such that the interconnects of each wafer are towardthe center of the stack. The wafers are then bonded together. All of theinterconnection between logic and control circuitry wafer 1850 and SPADwafer 1840 is accomplished in the middle of the stack (that is, at theface interface). Therefore, all of the interconnection metal arrayedacross the SPAD wafer 1840 surface does not shield the diffusions fromincident illumination. SPAD pixels have one wire connection per cell(e.g., anode), down to logic and control circuitry wafer 1850. All ofthe fine features (for the image sensor logic) are located at the logicand control circuitry wafer 1850. According to an embodiment, the onlyconnection that is made between the layers of the wafers is the anodeconnection. For example, a high voltage cathode connection at the top(SPAD) provides a connection corresponding to the high voltage supply ofan imaging sensor array, while low voltage connections correspond to rowenable, and read signals.

At step 1915 the bonded wafers 1840 and 1850 undergo backside grindingin order to thin the stack, with substrate removal from logic andcontrol circuitry wafer 1850 sufficient to expose TSV 1930, and supportwafer removal from SPAD wafer 1840. While TSV 1930 is shown, otherconnections (e.g., bond wires) are possible, with other bonding schemes.The pixel pitch of an imaging sensor array formed according toembodiments of the present disclosure is approximately 3 microns. Afterbackgrinding, a microlens array 1950 is attached to the backside of theSPAD wafer 1840, in order to increase fill factor of SPAD pixels inimaging sensor array 115. SPAD wafer 1840 is configured for backsideillumination. Advantageously, sensitivity to illumination in theinfrared spectrum is increased with a SPAD wafer 1840 oriented in themanner described herein, as sensitivity of a photosensitive element toinfrared wavelengths increases at greater depths from the incidentsurface. The thickness 1725 of the SPAD wafer 1840, which can be athickness of an epitaxially grown substrate, can be selected based onthe particular optical interrogation wavelength with which an opticalinterrogation system is configured to operate.

Referring now to FIG. 20, an imaging sensor array 115 b having a stackedwafer design and a reduced pixel pitch P′ in accordance with embodimentsof the present disclosure is depicted. Imaging sensor array 115 bincludes a wafer 2040 including SPAD cells with photosensitive regions145, the wafer 2040 stacked over a logic and control circuitry wafer2050. The imaging sensor array 115 b includes pixel having a reducedpitch P′, due to a shared HV n-wells and inter-cell n-diffusions. Thisis enabled due to the n-well having a constant HV input, along with thefact that only local junctions break down with direct photon incidence.Because image sensor array architectures according to the presentdisclosure enable global control of high voltage signals, HV n-wells andinter-cell isolation n-diffusions can be shared. An image sensoraccording to the embodiment of FIG. 20 has an n diffusion shared bymultiple p diffusions. This enables the imaging sensor arrayconfiguration shown, having pixel pitch P′ of, for example, 2-3 microns,further increasing the resolution capability of an imaging sensor in aranging device. Referring now to FIG. 20, an imaging sensor array 115 bhaving a stacked wafer design and a reduced pixel pitch P′ in accordancewith embodiments of the present disclosure is depicted. Imaging sensorarray 115 b includes a wafer 1840 including SPAD cells withphotosensitive regions 145, the wafer 1840 stacked over a logic andcontrol circuitry wafer 1850. The imaging sensor array 115 b includespixel having a reduced pitch P′, due to a shared HV n-wells andinter-cell n-diffusions. This is enabled due to the n-well having aconstant HV input, along with the fact that only local junctions breakdown with direct photon incidence. Because image sensor arrayarchitectures according to the present disclosure enable global controlof high voltage signals, HV n-wells and inter-cell isolationn-diffusions can be shared. This enables the imaging sensor arrayconfiguration shown, having pixel pitch P′ of, for example, 2-3 microns,further increasing the resolution capability of an imaging sensor in aranging device.

Referring now to FIG. 21, a flowchart 2100 of a method of fabricating asemiconductor device is depicted, in accordance with an embodiment ofthe present disclosure. The semiconductor device can be an imagingsensor (e.g., imaging sensor 115).

At step 2101 a first portion is formed in a first layer, the firstportion configured to operate at a first voltage. The first portionincludes a photodiode, for example, a SPAD.

At step 2103 a second portion is formed in a second layer, the secondportion configured to operate at a second voltage. The second voltagecan be different than the first voltage. According to an embodiment, thefirst voltage is higher than the second voltage (for example, the firstvoltage corresponding to a high-voltage CMOS wafer, the second voltagecorresponding to a low-voltage CMOS wafer). The second portion includeslogic and control circuitry configured to selectively forward an enablesignal to the photodiode in the first portion, and to receive anactivation from the photodiode.

At step 2105 the logic and control circuitry is electrically coupledwith the photodiode. According to an embodiment, the logic and controlcircuitry and photodiode are electrically coupled via damasceneinterconnects. According to an embodiment, logic and control circuitryand photodiode are electrically coupled via bond pads.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

What is claimed is:
 1. A sensor for measuring a distance to an object, the sensor comprising: an image sensor comprising an array of photodiodes, the image sensor operatively coupled to an interrogation source and configured to receive a reflected beam generated by illumination of the object by an interrogation beam of the interrogation source, and to develop a signal based on an array address of a photodiode of the array of photodiodes activated by incidence of the reflected beam on the array; a memory comprising a latch circuit configured to generate a set signal in direct response to the photodiode receiving the reflected beam, wherein the set signal reports the array address of the photodiode; an address decode circuit configured to store the array address; a timing circuit operatively coupled to the address decode circuit for reading the array address; and an analysis circuit operatively coupled to the interrogation source and to the image sensor, and configured to determine a distance to the object based upon the controlled angle of the photonic interrogation source and upon the array address.
 2. The sensor according to claim 1, wherein the address decode circuit comprises a memory configured to store column addresses of the array of photodiodes, and wherein each photodiode of the array of photodiodes is operatively coupled to a respective latch circuit.
 3. The sensor according to claim 2, wherein the timing circuit is configured to determine a state of the columns addresses sequentially, and further configured to withhold activation of timing of a read for a next column address until the column address corresponding to the reported array address is reached.
 4. The sensor according to claim 2, wherein each photodiode of the array of photodiodes is operatively coupled to a respective address decode circuit configured to store a respective column address, and wherein each row of the array of photodiodes is operatively coupled to a respective row latch circuit.
 5. The sensor according to claim 4, wherein the set signal is configured to deactivate other photodiodes of the array of photodiodes on a same row as the photodiode receiving the reflected beam.
 6. The sensor according to claim 4, wherein the timing circuit is configured to determine a state of the respective row latch circuits sequentially, and further configured to withhold activation of timing of a read for a next row latch circuit until the row latch circuit corresponding to the reported array address is reached.
 7. The sensor according to claim 1, wherein the array of photodiodes comprises avalanche photodiodes configured to operate in Geiger mode.
 8. The sensor according to claim 1, wherein the array of photodiodes comprises active photodiodes, and further wherein a photodiode of the array of photodiodes is activated when a voltage of the photodiode reaches a threshold level based on incident photons.
 9. The sensor according to claim 1, wherein the image sensor is disposed at a controlled distance from the interrogation source, and the analysis circuit is configured to determine the distance to the object based on triangulation of the controlled angle, the controlled distance, and an angle of incidence determined by the array address of the photodiode.
 10. The sensor according to claim 1, wherein a control timing circuit activates photodiodes of the array of photodiodes with a controlled activation period, based on a time-of-flight and a return angle of the interrogation beam to return to the image sensor at a maximum range.
 11. The sensor according to claim 8, wherein the sensor further comprises an ambient light sensor configured to detect an ambient light level, and further wherein the controlled activation period is based on a detected ambient light level.
 12. A pixel circuit, comprising: a photodiode; a load comprising at least one of an active and a passive element, electrically coupled to the photodiode; and a plurality of transistors electrically coupled to the photodiode, the plurality of transistors comprising a latch circuit; wherein a control of the photodiode is referenced to ground and configured to selectively activate the photodiode, wherein the photodiode is configured to develop a current based on photons incident at the photodiode, the load configured to convert the current to a voltage pulse, the voltage pulse configured as an input to set a logic level of the latch circuit.
 13. The pixel circuit according to claim 12, wherein the pixel circuit further comprises a voltage clamp circuit electrically coupled to the photodiode and to the control, the voltage clamp configured to limit a voltage level of the voltage pulse input to the latch circuit.
 14. The pixel circuit according to claim 12, wherein the photodiode is an avalanche photodiode configured to operate in Geiger mode, and the current is an avalanche current triggered by the photons.
 15. The pixel circuit according to claim 12, wherein the photodiode is an active photodiode, and further wherein the current is developed when a voltage of the photodiode reaches a threshold level based on the photons incident at the photodiode. 